mm 


m 

M' 

*l«ili«< 

im 

mmm< 


(Ulif  i.  1.  Bill  ICtbrarg 


5Jnrtb  (Earoltna  S>lat?  Colkge 

QH211 
cop.  2 


1*1,^;  SyATE  u\;vE»s;-.- 


0  H     H 


'LL   LIBRARY 


SOOl 40377 


M 


1826 

This  book  is  due  on  the  date  indicated  below 
and  is  subject  to  a  fine  of  FIVE  CENTS  a 
day  thereafter. 


MANIPULATION 


OF   THE 


MICROSCOPE, 


— BY — 


EDWARD  BAUSCH. 


ILLUSTRATED, 


PUBLISHED     BY     BAUSCH    &     LOME     OPTICAL     CO. 


ROCHESTER,  N.  Y.  : 

POST-EXPRESS  BOOK  AND  JOB  PRINTING  HOUSE. 

1885. 


<^ 


Copyright. 

Edward    Bausch. 

1885. 


^w 


/ 


TO    THE    READER. 


It  may  seem  to  some  persons  an  act  of  presumption 
for  a  maker  of  microscopes  and  microscope  accessories 
to  enter  the  field  of  authorship  and  attempt  to  supple- 
ment the  valuable  labors  which  in  recent  years  have 
made  the  use  of  the  microscope  an  indispensable  aid 
in  the  advancement  of  science. 

To  such,  if  any,  I  submit,  that  being  a  producer  of 
microscopes  and  their  accessories,  I  have  had  oppor- 
tunity to  become  acquainted  with  the  lack  of  general 
knowledge  of  the  fundamental"  principles  of  the  instru- 
ment and  the  best  method  of  technique^  even  among 
owners  of  microscopes.  Indeed,  with  so  many  com- 
plications, with  almost  unlimited  powers  and  uses  of 
the  instrument,  the  beginner  cannot  fail  to  feel  the 
need  of  a   guide  and  adviser. 

In  order  to  accomplish  the  greatest  good,  I  have 
started  out  in  this  little  Mtmual  with  the  supposition 
that  the  purchaser,  or  owner,  is  a  beginner  and  abso- 
lutely ignorant  of  the  microscope  and  everything  which 
pertains  to  it,  and  therefore  have  attempted  to  convey, 


^"5-^ 


step  by  step,  in  as  simple  language  as  I  could  command, 
information  which  will,  I  trust,  lead  to  ease  of  manipu-i 
lation  and  give  both  pleasure  and  profit  to  those  for 
whom  it  was  specially  written. 

With  these,  its  purposes  and  hopes,  I  beg  for  my  self- 
imposed  labor  a  friendly  reception. 

Edward   Bausch. 


CONTENTS. 


Simple  Microscopes. 

Purpose  of  the  Microscope ;  Kinds  of  Microscopes ; 
Magnifying  Power;  Using  Magnifiers;  Aberrations; 
Achromatism,  -  -  -  -  -  7 

The  Compound  Microscope. 

Description  of  Parts ;  Base  or  Foot ;  Pillar ;  Arm ; 
Body ;  Nose-Piece ;  Society-Screw ;  Eye-Piece,  or 
Ocular  ;  Draw-Tube  ;  Collar ;  Coarse-Adjustment ; 
Milled  Heads;  Fine- Adjustment ;  Stage;  Clips;  Cen- 
tering Screws;  Mirror;  Mirror-Bar;  Sub-Stage; 
Sub-Stage  Bar;  Diaphragm;  Optical  Axis;  Object; 
Slide ;  Cover-Glass,  -  -  -  -         12 

Objectives  and  Eye-Pieces. 

Objectives-Classes  ;  Systems  ;  Angular  Aperture  ;  Achro- 
matism ;  Resolving  or  Defining  Power ;  Flatness  of 
Field ;  Penetration  ;  Working-Distance  ;  Magnifying 
Power  ;  Selecting  Objectives  ;  Eye-Pieces — Huyghen- 
ian;  Solid;  Periscopic ;  Flatness  of  Field;  Size  of 
Field,  ---._-        17 

Requisites  for  Work. 

Working  Table;  Room;  Light;  Position  of  Light; 
Which  Eye  to  Use;   Order,  -  -  -        40 

How  TO  Work. 

To  Set  Up  the  Instrument ;  Centering  Stage  ;  Illumina- 
tion ;    Attaching    High    Power    Objectives ;    Double 


6 


Nose  Piece ;  How  to  AVork ;  Dark  Ground  Illumina- 
tion ;  Polarized  Light ;  Cover-Glass ;  To  Draw ; 
Camera  Lucida — How  to  Use  It ;  Determining  the 
Magnifying  Power,  -  -  -  -        45 

Advanced  Manipulation. 

Dry- Adjustable  Objectives  ;  Immersion- Adjustable  Ob- 
jectives; Test-Plate;  Immersion  Objectives  on  Test- 
Plate,  -  -  -  -  -  -        66 

Sub-Stage  Illumination. 

Objectives  and  Eye- Pieces  ;  Hemispherical-Lens ;  Wen- 
ham  Button ;  Woodward  Prism  ;  Narrow  Angle  Con- 
densers ;  Wide  Angle  Condenser ;  W^ard's  General 
Illuminator ;  Large  Sub-Stage  Condenser,    -  -         79 

Care  of  a  Microscope. 

To  Take  Care  of  a  Stand ;  To  Take  Care  of  Objectives 
and  Eye-Pieces,         -  -  -  -  -         85 

Considerations  in  Testing  Objectives,    -  -  89 


SIMPLE    MICROSCOPES. 


Purpose  of  the  Microscope.— The  Microscope  is 
an  instrument  which  magnifies  objects,  so  that  we  are 
better  able  to  examine  their  structure  than  is  possible 
with  unassisted  vision. 

Kiocls  of  Microscopes.— Microscopes  may  be  di- 
vided into  two  classes — simple  and  compound — the  dif- 
ference between  the  two  being  that  with  the  forrner,  the 
object  is  viewed  directly,  while  with  the  latter  a  magni- 
fied image  is  observed :  while  the  first  shows  the  objects 
in  their  true  position,  the  latter  shows  them  reversed,  so 
that  what  is  right  in  the  object  is  left  in  the  image,  and 
when  an  object  appears  to  be  moving  in  a  certain  direc- 
tion, the  movement  is  in  reality  the  reverse,  and  must  be 
moved  accordingly  to  keep  it  in  view. 

Magnifiers.— Simple  microscopes  are  usually  termed 
magnifiers,  and,  whether  consisting  of  one  or  more  lenses, 

always      remain 

s  i  m  pie.       The 

most       common 

are    those    with 

one    or    several 

double-convex 

Fig.  I.  lenses,     (Fig.  i.) 

The  shorter  the  radii  (the  more  curved  the  surfaces)  are 

in  these,  the  greater   will  be  the  magnifying  power,  and 

the  higher  this  is,  the  less  of  the  object's  surface  can  be 


4:. 


>/ 


^^^JtAsr 


♦  •^tcte  r^i, 


8 


seen  at  once.  Each  additional  lens  increases  the 
magnifying  power  in  proportion  to  its  curvature. 
The  distance  between  the  lens  and  the  object,  when 
this  is  seen  most  distinctly,  is  called  the  focus;  at  the 
point  where  the  object  is  most  distinct,  the  lens  is 
said  to  be  ///  focus;  when  indistinct  or  blurred,  out 
of  focus. 

Magnifying  Power. — Unless  a  microscope  is 
known  to  come  from  the  hands  of  a  reliable  firm, 
any  claim  as  to  magnifying  power  should  be  accepted 
with  reserve.  In  former  years,  when  the  country  was 
over  run  with  cheap  foreign  productions,  the  most  fanci- 
ful claims  were  made  in  this  direction.  It  is  evident 
that  a  lens  magnifies  an  object  equally  in  all  directions ; 
this  is  said  to  be  in  areas,  and  is  the  square  of  the 
linear,  so  that  if  an  object  is  magnified  4  times  ia  the 
hnear,  it  is  16  times  in  area.  The  commonly  accepted 
term  to  express  magnifying  power  of  simple,  as  well  as 
compound  microscopes,  is  in  diameters  {li?iear).  A  single 
lens  of  I  inch  focus  magnifies  about  ten  diameters ;  one 
of  2  inch  focus,  about  5  diameters  ;  one  of  ^  inch  focus, 
20  diameters,  and  so  on.  In  a  lens  of  high  magni- 
fying power,  the  focus  is  ordinarily  about  twice  the 
diameter,  so  that  if  a  lens  is  ^  inch  diameter  its  focus 
is  about  I  inch.  This  may,  however,  be  more  accu- 
rately determined  by  projecting,  say  a  flame  or  window - 
frame,  upon  a  white  piece  of  paper ;  the  distance 
between  the  paper  to  the  center  of  the  lens,  when  the 
image  is  most  distinct,  is  its  focal  distance.  When  a  lens 
is  two  inches  or  more  in  diameter,  it  is  usually  termed  a 
reading  glass. 


9 


Using  Mag-uiflers.— In  using  magnifiers,  the  lens 
should  be  held  close  to  the  eye  and  such  a  position 
taken  that  the  object  will  receive  the  best  illumination. 
In  the  lenses  of  equally  convex  surfaces,  it  is  immaterial 
which  side  is  held  toward  the  eye;  but  when  plano- 
convex lenses  are  used,  the  plane  side  should  always  be 
toward  the  eye,  as  it  gives  the  flattest  field. 


Aberrations.  -  Two  factors  arise  which  prevent  the 
advantageous  use  of  more  than  about  25  diameters  in 
magnifiers ;  they  are  called  the  chromatic  and  spherical 
aberrations.  The  first  is  the  term  employed  when  the 
object  is  apparently  fringed  with  color,  predominently 
blue  and  yellow ;  the  second,  when  all  but  the  central 
portion  of  the  lens  shows  the  object  indistinctly  ;  these 
faults  increase  with  the  magnifying  power.  In  the  case 
of  a  combination  of  several  lenses,  they  may  be  partially 
overcome  by  interposing  an  opaque  plate  with  a  small 
opening,  called  a  diaphragui^  between  them, 
which  cuts  off  the  outer  or  marginal  rays, 
or  the  lenses  may  be  made  of  a  smaller 
diameter.  An  incision  m^ay  also  be  cut 
into  the  glass  equally  between  the  two 
surfaces,  when, 
name  of  the  in- 
is  called  a  Cod- 
2     shows 


Fig.  2. 

from    the 

ventor,   it 

dington. 

section     of     a    Coddington, 

while    Fig.    3    shows    it    in 

its  mounting. 


Fior 
J.  1^. 


a 


Fig.  3. 


10 


Ai'liroiiiatisin.— Tlic  most  approved  method,  how- 
ever, for  ehminating  these  a])pearances  is  by  the  use  of 
one  or  two  concave  fliut  glass  lenses  in  connection  with 
the  double  convex  cnncm  g/ass  lens.  When  the  color  or 
chromatic  aberration  is  thus  removed,  the  lenses  are 
said  to  be  achromatic^  and  when  both  the  chromatic  and 
spherical  aberrations  are  avoided,  the  lens  is  called 
aphmatic,  and  is  then  said  to  be  coj^rected.  An 
achromatic  lens,  composed  of  one  flint  and  one  crown 
glass  lens,  is  called  a  doublet ;  (Fig.  4) 
one  with  two  flint  glass  lenses  and  one  of 
crown  glass  is  called  a  t7'iplet.  Fig.  5.) 
Fig.  4-  The  latter  is  the  best  form,  as  it  gives  the 

highest    correction ;   such  a  lens  (it  is  thus 
called    from    the   fact    that    the    lenses    are   V 
cemented  together  and  act  like  one)  may  be 
held  with  either  side  toward  the  object  with 
equally  good  results,   and  may  also  be  held 
at  quite  an  obliquity,  without  loss  of  defini- 
tion ;  this  feature  is  important,  as  it  is  almost  impossible 
to  give  a  lens  a  theoretically  correct  position  to  both  the 
eye  and  object  with  the  unaided  hand. 


Fig.  5- 


11 


■'^^'^J''Ke,irj-M/>/,,9i^oy/.;. 


Fig.  6. 


THE    COMPOUND    MICROSCOPE. 


As  was  previously  stated  a  magnified  image  is  ob- 
served in  the  Compound  Microscope.  Any  two  lenses, 
one  of  short,  the  other  of  a  long  focus,  placed  sufficient- 
ly far  apart,  will  attain  this  object,  and  this  was  for 
years  the  method  of  its  construction.  It  was  found 
that  on  account  of  the  extreme  magnifying  power, 
mechanical  appliances  were  required  to  keep  the  lenses 
steady  during  observations  and  at  their  proper  dis- 
tances, also  that  provision  was  necessary  for  adjust- 
ment and  illumination.  These  adjuncts  were  so  neces- 
sary that  the  instrument  could  not  be  conceived 
without  them,  and  the  entire  apparatus  is  thus,  by  the 
force  of  usage,  called  a  miscroscope,  and  the  instru- 
ment without  the  optical  parts,  a  stand. 

Oescription  of  Parts.— As  it  is  necessary  for  the 
student  to  become  conversant  with  the  terms  of  the 
various  parts  and  to  understand  their  use,  we  give  an 
illustration  (Fig.  6)  with  letters,  and  append  a  list 
giving  the  names. 

A.  Base  or  Foot.— This  is  the  foundation  of 
the  instrument.  It  usually  rests  upon  three  points 
(or  should  do  so)  and  is  of  such  a  weight  that  it 
keeps  the    instrument   firm    when  it    is   in    an    upright 


13 

or  inclined  position.  The  revolving  plate,  when  this  is 
provided,  by  means  of  which  the  upper  portion  of  the 
instrument  is  revolved,  without  changing  the  position 
of  the  base,  is  considered  a  part  of  it. 

B.  Pillar.— It  is  that  portion  which  is  fastened  to 
the  base  and  may  be  one  or  two,  according  to  the 
construction  of  the  stand.  It  carries  upon  its  upper 
end  t\iQ  Joint  or  axis. 

C.  Ann.— This  is  connected  with  the  pillar  by  the 
joint  and  supports  all  the  working  parts  of  the  in- 
strument. 

D.  Body.— This  is  the  tube-portion  to  which  the 
optical  parts  are    attached. 

E.  Nose-piece.— This  is  an  extra  piece  which  is 
attached  to  the  lower  part   of  the  tube. 

Society  Screw.— This  is  a  standard  screw  which  is 
cut  into  the  nose-piece,  and  is  called  so  from  the  fact 
that  it  was  first  established  by  the  Royal  Microscopical 
Society  of  London.  It  is  also  called  the  imiversal 
screw,  and  is  in  general  use  in  this  country  and  England  ; 
it  has  lately  been  adopted  by  some  firms  on  the  Con- 
tinent of  Europe. 

F.  Objective.— This  is  screwed  into  the  nose- 
piece  and  is  called  so  because  it  is  nearest  the 
object.  It  is  the  most  important  of  the  two  opti- 
cal parts  (of  the  microscope  proper)  and  upon  its 
perfection  the  distinctness  of  the  image  and  therefore 
the  value  of  the  instrument  almost  entirely  depends. 


14 

G.  Eye-piece  or  Oeular.— It  is  called  so  because 
it  is  nearest  the  eye  and  is  the  remaining  optical 
part.  It  magnifies  the  image  given  by  the  objective. 
This  and  objective  will  be  treated  more  fully  later 
on. 

H.  Draw-tube.— This  is  that  portion  of  the  body 
which  moves  in  the  outer  sheath  and  which  receives 
the  eye  piece.  It  is  provided  for  the  purpose  of  attain- 
ing variations  in  magnifying  power  and  as  a  matter  of 
convenience  while  working. 

I.  Collar.— This  is  a  ring  which  is  attached  to  the 
draw-tube    and  is  usually  provided    with  a  milled  edge. 

J.  Coarse  Artjiirstineiit.— This  is-  a  provision  for 
moving  the  body  quickly  back  and  forth  for  ad- 
justing the  focus  a])proximately.  It  is  done  by  a 
sliding  rack  and  stationary  //;//^//  (not  shown  in  cut) 
or  a  sliding  body  in  an  outer  sheath. 

K.  Milled  Heads.  These  are  attached  to  the 
shank  of  the  pinion,  which  is  revolved  by  means  of 
them  and  are  usually  large  to  give  sensitiveness  to  the 
movement. 

Li.  Fine  Adjustinent.— This  is  slow  moving  and 
serves  to  get  an  exact  focus.  It  is  attained  by  a  fine 
thread,  provided  with  a  milled  head,  and  acts  upon 
the  body,  either  directly  or  by  levers.  This  as  well 
as  the  coarse  adjustment  should  be  extremely  sensi- 
tive and  should  not  have  the  least  side  or  lateral 
motion.  The  fact  that  either  of  them  have  it,  is  evi- 
dence of  poor  workmanship. 


15 


M.  Stag^e. — This  is  the  portion  on  which  the  ob- 
ject is  placed  for  examination.  It  is  attached  to  the 
arm  and  may  be  either  permanently  fixed,  or  revolving  or 
mechanical,  the  latter  by  moving  the  object  by  me- 
chanical contrivances  instead  of  by  fingers.  Metal  and 
glass  are  used   in  its  construction. 

X.  Clips. — These  are  two  springs  which  are  at- 
tached to  the  upper  surface  of  the  stage  and  serve  to 
hold  down  the  object. 

Centering  Screws. — These  are  provided  for  mov- 
ing the  stage  in  different  directions  to  bring  the  center 
of  its  revolving  motion   in  the  center  of  the  field. 

O.  Mirror.— This  is  used  for  reflecting  and  con- 
densing light  upon  the  object.  As  a  rule  two  are 
used,  one  plain  and  the  other  concave.  The  first 
gives  a  comparatively  weak  light,  while  the  second 
concentrates  it   and  gives  it  more  intensity. 

P.  Mirror-I>ar. — Tliis  carries  the  mirror  and  by  a 
sliding  arrangement  allows  the  variations  in  distance 
of  the  mirror  to  the  stage  ;  it  also  swings  in  a  circle 
around  the  object  in  order  to  illuminate  it  from  any 
direction. 

Q.  Sul>-sta8"e.— This  is  a  ring  below  the  stage  to 
receive  various  accessories  which  may  be  required. 
It  is  sometimes  fixed  to  the  stage  but  in  the  best  in- 
struments it  is  separated  from  it  and  is  provided  with 
an  adjustment  to  vary  its  distance  from  the  object. 


16 


R.  Sub-stage  bar.— This  receives  the  sub-stage 
and  permits  its  adjustment.  This  as  well  as  the  mir- 
ror-bar should  be  on  an  axis  in  the  plane  of  the 
stage,  so  that  whatever  position  they  may  be  in,  rela- 
tive to  the  object,  the  distance  from  this  to  the  sub- 
stage  or  mirror  does  not  vary,  except  when  made  to 
do    so. 

S.  Diapbrag-m.  -This  is  a  perforated,  revolving 
disk,  attached  either  to  the  stage  or  sub-stage.  It 
has  holes  of  different  sizes  so  that  the  amount  of 
light    from    the    mirror    may   be    modified. 

Optical  Axis.— This  is  an  imaginary  line  which  passes 
from  the  center  of  the  eye-piece  through  the  body, 
objective,  stage  and  sub-stage  to  the  mirror.  What- 
ever lies  in  it  is  said  to  be   centered. 

Object. — Is  that  which .  is  examined  and  placed 
upon  a 

Slide.— This  is  a  thin  plate  of  glass,  generally  3 
inches  long   by    i    inch  wide. 

Cover  Glass.— This  is  an  extremely  thin  piece  of 
glass,  round  or  square,  which  is  placed  upon  the  ob- 
ject, either   for    flattening   or   preserving   it,  or  both. 


OBJECTIVES   AND    EYE-PIECES. 


Although  considerable  magnifying  power  may  be 
attained  by  the  use  of  two  single  lenses  arranged  in 
a  compound  form,  there  is  no  advantage  in  it,  from 
the  fact  that  the  faults  in  the  lenses  are  correspond- 
ingly magnified,  and  these  are  so  considerable  that 
they  destroy  what  it  is  the  .  purpose  of  the  micros- 
cope  to  give — a  distinct  image. 

Objectives,  Classes.- Objectives  may  be  divided 
into  two  classes,  dry  and  immersion  ;  in  the  former  no 
intervening  medium  except  air  exists  between  the 
cover  and  objective,  while  in  the  latter  a  fluid  is  used 
to  connect  the  upper  surface  of  the  cover  to  the 
front  surface  of  the  object.  The  use  of  immersion 
fluid  has  several  advantages,  the  first  of  which  is  that 
the  objective  may  be  made  to  give  better  perform- 
ance, as  will  be  explained  later  on ;  the  second  is  that 
more  light  will  be  transmitted,  as  there  is  less  loss  of 
it  by  refraction.  It  must  be  observed  that  an  ob- 
jective which  is  made  to  use  dry  can  not  be  used  with 
immersion,  nor  vice  versa. 

There  are  two  fluids  in  general  use  at  the  present 
time,  water  and  homogeneous  fluid.  The  latter  expres- 
sion means  of  the  sajne  kind,  and  refers  to  the  fact  that 


18 


the  fluid  has  about  the  same  refractive  and  dispersive 
power  as  glass,  so  that  when  this  fluid  fills  up  the 
space  between  the  two  surfaces  of  glass,  a  ray  of  light 
passes  through  the  three  mediums  as  if  they  were 
one  body. 

Objectives  are  sometimes  called  pou'ers,  and  in  this 
sense  are  divided  into  three  classes :  /oiv,  mediinn 
and  kig/i.  Dr.  Carpenter  classifies  them  as  follows : 
low  _p07uers,  3  inch,  2  inch,  i^  inch,  i  inch,  f  or 
^  inch  ;  medium  powers^  1,,  inch,  \  inch,  ^  inch, 
i  inch  ;  high  powers^  \  inch,  \  inch,  i^,  inch,  h  inch, 
tV  inch,   2V  inch,   oV  inch. 

As  the  objective  is  the  most  important  of  the 
two  optical  parts,  it  follows  that  this  must  be  as  free 
from  faults  as  possible  and  all  that  human  ingenuity 
and  skill  can  devise  is  utilized  to  attain  this  end.  The 
advance  in  the  perfection  of  the  objective  has  been  step 
by  step  and  each  era,  was  at  the  time,  considered  by 
many  authorities  the  limit  to  further  improvement. 
Each  advance  was  signalized  by  a  marked  opposition 
and  disbehef  of  its  possibifity.  It  is  therefore  of  in- 
estimable credit  to  the  pioneer  objective-makers,  and 
notably  among  these  two  Americans,  who  by  quiet 
but  stubborn  appfication,  disproved  previous  claims 
and  opened  the  way  to  further  improvements.  A 
theoretical  fimit  has  been  fixed  on  the  capacity  of 
the  microscope,  which,  according  to  our  present  know- 
ledge can  not  even  be  reached,  but  it  is  still  safe 
to  say  that  the  end  is  not  yet.  A  modern  objective 
of  the    highest   capacity   may   be    considered    a    work 


19 


of  art,  and  there  are  few  productions  of  the  human 
hand  which  exact  so  much  untiring  application,  in- 
genuity  and   skill. 

Systems.— An  objective  is  said  to  consist  of  sys- 
tems which  may  vary  in  number  from  one  to  four 
and  five ;  two  and  three  are  however  mainly  in  use. 
They  are  the  individual  portions  consisting  of  one, 
two  or    three   lenses,  which   when   more   than   one,   are 

cemented  together  and  make 
up  the  objective.  An  achro- 
matic single  system  may  con- 
sist of  two  or  three  lenses, 
and  a  three  or  four  system 
objective  may  consist  of  as 
many  as  seven  or  eight  lenses. 
The  systems  are  called  in  their 
order :  anterior  or  front,  mid- 
dle and  posterior.  When  one 
consists  of  two  lenses  it  is  called  a  doublet,  when  of  three 
lenses  a  triplet.  Thus  in  Fig.  7,  A  is  the  anterior, 
M  the'  middle  and  P  the  posterior  systems  ;  thus  also 
A  is  a  single  system,  M  a  double  and  P  a  triple  one. 

The  various  features  which  must  be  considered  as 
determining  the  quality  of  an  objective  are :  angular 
aperature,  achromatism,  resolving  or  defining  power, 
flatness  of  field,  penetration,  working  distance  and 
magnifying  power.  Although  these  attributes  may  be 
considered  separately,  some  of  them  go  hand  in  hand. 
The  presence  or  extent  of  one  necessarily  involves 
or   precludes  another. 


Fig.  7. 


20 


Aiig-ular  Aperture.— The  angle  which  the  most 
extreme  rays,  which  are  transmitted  through  the  ob- 
jective, make  at  the  point  of  focus,  is  called  its 
angular  aperture,  or  in  short  its  angle,  and  of  all 
the  qualities  in  an  ideal  objective,  this  is  the  most 
important.  Thus  in  Fig.  8,  D  is  considered  the 
point  of  focus,  and  C  D  E  the  angular  aperture. 
The  above  definition  has  its  limi- 
tations, however.  While  in  ob- 
jectives of  proper  construction  it 
holds  true,  there  are  many  in 
which  it  is  not  the  case.  For  in- 
stance, an  objective  may  be  so 
constructed  that  it  may  transmit 
a  considerable  number  of  rays  in 
excess  of  those  which  combine  to 
form  an  image,  and  it  is  evident 
that  these  should  not  be  considered 
as  belonging    to   them. 


As  there  are  many  objectives  of  the  same  power,  but 
of  different  angular  aperture,  there  are  again  others  of 
varying  power,  but  of  the  same  angle.  Other  things 
being  equal,  it  is  the  angular  aperture  of  an  objective 
which  determines  the  quality.  It  is  expressed  in 
degrees^  and  is  also  spoken  of  as  being  wide,  medium  or 
narrow,  although  this  is  indefinite  and  depends  consider- 
ably upon  the  power  of  the  objective;  while  the  angle 
may  be  excessively  wide  for  a  low  power,  it  may  l)e  nar- 
row for  a  higher  one. 


21 


For  many  years  the  extent  to  which  angular  aperture 
could  be  carried  was  a  matter  of  controversy,  as  was 
also  the  use  of  objectives  of  wide  and  narrow  angles  for 
different  directions  of  work.  It  is,  however,  a  matter  of 
congratulation  that  the  question  is  at  rest,  although  it 
has  served  a  good  purpose  in  promulgating  a  better 
knowledge  of  the  subject. 

All  objects  emit  rays,  and  it  is  evident  that  those 
coming  from  one  point  and  contained  in  a  large  angle 
are  more  numerous  than  those  in  a  small  angle  ;  also, 
that  as  the  angle  more  nearly  approaches  i8o^,  the  rays 
will  not  only  be  larger  in  number,  but  will  embrace 
more  of  the  surface  of  the  point ;  if  we  imagine  the 
point  to  be  a  sphere  or  globule,  it  is  apparent  that  we 
see  its  upper  half  more  distinctly,  when  in  addition  to  a 
top  view  we  also  get  a  side  view,  than  when  we  get  a  top 


A - 


E     ...  'C  A 


Kig.   y. 


22 


view  only.  As  this  is  true  with  the  unassisted  eye,  it  is 
also  a  fact  with  the  microscope.  This  is  shown  in 
Fig.  9.  It  is  assumed  that  two  objects,  B  and  B',  are 
equally  bright,  and  therefore  emit  the  same  number  of 
rays  ;  for  the  purpose  in  hand,  it  is  sufficient  to  consider 
only  those  which  reach  the  plane  surface  of  the  large 
lens  ;  if  an  equal  space  is  imagined  over  B',  the  same 
number  of  rays  will  be  contained  in  this  ;  therefore,  the 
cone  contained  in  the  angle  A'  B'  C  will  contain  as 
many  rays  as  that  contained  in  A  B  C ;  but  as  the  lens 
D'  E'  is  considerably  smaller  than  A  C,  only  as  many 
rays  can  enter  it  as  are  contained  in  the  angle  D'  B'  E'. 
As  the  rays  contained  in  the  angle  ABC  and  D'  B'  E' 
are  carried  through  the  two  lenses,  which  are  supposed 
to  be  of  the  same  magnifying  power,  the  image  formed 
by  A  C  will  be  considerably  brighter  than  that  formed 
by  D'  E',  and  will  therefore  show  more  of  its  structure, 
as  will  be  shown  hereafter.  .Besides  this,  the  rays  con»- 
prised  in  the  angle  ABC  cover  more  of  the  object's 
surface  than  do  those  of  the  angle  I)'  B'  E'. 

It  is  therefore  indisputable  that  objectives  of  the  same 
angular  aperture,  butof  dift'erent  magnifying  power  (within 
ordinary  limits),  will  show  the  object  equally  well,  pro- 
vided they  are  otherwise  of  the  same  quality,  and  also 
true  that  in  objectives  of  the  same  power  but  unecpial 
angular  aperture,  the  one  of  wider  angle  will  sliow  an 
object  more  brilliantly  than  the  other,  and,  if  the  differ- 
ence be  considerable,  will  show  structure  of  which  no 
trace  can  be  found  with  tlie  narrow  angle.  These  are 
facts  which  are  based  upon   natural  laws,  but  there  are 


23 


other   conditions    to  l)e  considered  in  connection   witli 
them,  which  will  be  treated  hereafter. 

It  very  often  happens  that  objectives  from  different 
makers,  but  of  the  same  angle,  show  a  considerable 
variation  ;  this  does  not  prove  that  the  above  principle 
is  wrong,  but  is  evidence  that  greater  care  or  skill  has 
been  bestowed  on  one  than  on  the  other. 

By  arranging  an  objective  with  an  imine?'sio/i  front, 
its  angular  aperture  may  be  considerably  increased  over 
that  of  a  djy  front,  and  this  explains  why  better  results 
may  be  obtained  with  the  former  than  with  the  latter. 

Achromatisiu. — As  has  been  stated  before,  when 
single  lenses  are  made  to  give  a  high  magnifying 
power,  the  chromatic  and  spherical  aberrations  pre- 
vent corresponding  advantages ;  and  as  the  objective 
gjves  the  image  which  is  magnified  by  the  eye- 
piece, it  is  evident  that  if  they  exist  in  it,  they  are 
increased  by  the  ocular,  and  that  especial  care  must 
be  given  to  exclude  all  faults  as  much  as  possible 
from  it.  Even  with  the  use  of  flint  glass,  it  is  im- 
possible to  free  the  objective  entirely  from  color ; 
there  will  remain  a  residue  of  green  and  purj)le,  and 
these  colors  will  fringe  the  object.  These  are  called 
the  secondary  spectrum,  and  their  presence  in  an  ob- 
jective  is   usually   evidence    of  the    highest  correction. 

The  amount  of  color  in  an  objective  depends 
somewhat  upon  the  power  of  the  eye-piece,  and  be- 
comes more  visible  as  a  higher  power  is  used. 
Color  outside  of  the  secondary,  spectrum  is  not  always 


24 


])rejiulir,ial  to  an  objective  ;  for,  if  in  two,  one  shows 
the  structure  of  an  object  with  a  sHglit  amount  of 
it,  the  other  does  not  show  the  structure  but  gives 
a  nearly  '  colorless  image,  it  goes  without  argument 
to  sav  that  the  first  2:ives  the  best  results  and  is 
therefore  preferable. 

If  on  increasing  the  distance  between  the  objec- 
tive and  object  the  latter  shows  a  marked  bluish 
color,  and  when  the  distance  is  decreased  a  yellow- 
ish-red color,  the  objective  is  chromatically  iiiider-cor- 
rected ;  if,  however,  the  conditions  are  reversed — if  the 
object  shows  a  yellow  color  when  the  distance  be- 
tween it  and  the  objective  is  increased,  and  blue  when 
decreased,  it  is  over-corrected.  If  tlie  colors  are  not 
sufficiently  i)ronounced  to  judge  the  corrections,  they 
may  be  determined  more  positively  by  means  of  ob- 
lique light.  If,  when  the  mirror  is  swung  to  the  right, 
the  right  edge  of  the  object  is  yellow  and  the  left 
edge  blue,  the  object  is  chromatically  over-cor?  ected ; 
if  the  right  edge  is  blue  and  the  left  yellow,  it  is 
imder-corrected. 


Resolving:    or    Defining    Power.— This    is    the 

quality  in  an  objective  by  which  we  are  enabled  to 
see  the  intricate  structure  and  finer  details  in  an 
object.  It  depends  upon  the  correctness  of  the 
chromatic  and  spherical  aberrations,  upon  angular 
aperture,  and  of  course  upon  the  perfection  of  the 
mechanical  work.  The  power  of  resolving  in  an 
objective  is  indicative  of   the    perfection  of  the  micro- 


25 


scope,    for   it    is    almost    entirely    dependent    upon    it 
for  its  ([Liality. 

When  an  objective  is  said  to  feso/ve  a  structure 
or  a  certain  number  of  lines,  it  means  that  it  shows 
them  under  certain  conditions  of  light.  It  may  re- 
solve easily  or  only  glimpse  them- — ^the  latter  when 
they  are  hardly  to  be  distinguished.  Every  degree 
added  to  the  angular  aperture  of  an  objective  in- 
creases its  resolving  power,  and  the  theoretical  capa- 
city of  every  degree  has  been  mathematically  deter- 
mined. However,  this  standard  is  only  reached 
approximately  and  to  a  varying  extent.  It  is  not  by 
any  means  said  that  every  objective  of  a  certain 
angular  aperture  will  have  a  corresponding  resolving 
power  ;  it  is  at  this  point  that  the  acute  accuracy  of 
work  and  superior  judgment  of  proper  corrections 
will    invariably   give    the    best    results. 

It  is  an  error  to  suppose  that  the  resolving  power 
may  be  improved  by  merely  increasing  the  magnify- 
ing power.  It  is  an  invariable  quality  of  an  objective 
and  has  a  fixed"  hmit.  The  extent  to  which  it  may  be 
approached  depends  upon  the  nicety  of  manipulation, 
but  no  amount  of  increase  in  magnifying  power  by 
the  eye-piece  or  any  other  means  will  carry  it  beyond 
it  ;  on  the  contrary,  it  will  lose  in  this  respect  if 
carried   beyond    a    certain    point. 

Flatness  of  Field.— The  Jie/d  in  a  microscope 
is  that  portion  which  is  observed  in  the  eye  piece, 
and  its  flatness  may  be  observed  when  focused  on 
a  flat    object — preferably  a  micrometer.     It  is  said  to 


26 


be  flat  when  all  portions  of  the  object  are  seen  over 
the  entire  field  at  once  without  further  focusing. 
When  not  flat,  it  will  be  found  that  as  the  image 
approaches  the  edge  of  the  field  it  becomes  more 
and    more    indistinct,  and  that  the    objective    must  be 

correspondingly  adjusted  ;  in  many 
cases  it  remains  indistinct  or  blur- 
red, and  this  may  be  considered 
the  most  serious  fault.  In  the 
case  of  looking  at  straight  parallel 
lines,  such  as  in  a  micrometer, 
they  will  appear  to  become  more 
curved    as    they    near    the    edge, 

lO. 


Fii^.   lo. 


as  shown    in    Fig 


Flatness  of  field  mainly  depends  upon  the  correc- 
tion of  the  spherical  aberrations,  and  as  under  the 
best  conditions  the  latter  •  cannot  be  entirely  elimin- 
ated, it  is  impossible  to  attain  absolute  flatness,  except 
with  eye-pieces  especially  made  for  this  purpose.  It 
may,  however,  also  be  due  to  a  faulty  eye-piece  ;  in 
this  case  it  can  fairly  be  determined,  by  observing 
whether  it  shows  equally  in  different  objectives.  With 
beginners,  especially,  it  is  usually  most  complained  of, 
owing  probably  to  the  fact  that  it  is  the  most  easily 
noticeable.  Although  a  desirable  quality,  we  con- 
sider it  a  matter  of  minor  importance,  and  in  the 
choice  between  two  objectives,  in  one  of  which  the 
predominant  feature  is  resolving  power  and  in  the 
other  flatness  of  field,  our  choice  would  invariably  be 
for    the    former. 


27 


Penetration. — This  is  the  quahty  which  enables 
us  to  look  into  an  object — to  observe  different 
planes  at  one  time  In  the  mind  of  the  writer,  it 
is  of  no  special  importance,  or,  at  any  rate,  not  as 
much  as  is  claimed  for  it,  and.  if  desired,  is  easily 
attained.  It  depends  upon  magnifying  power  and 
angular  aperture,  and  decreases  with  the  increase  of 
either  of  these.  Objectives  are  generally  not  con- 
structed with  any  reference  to  it ;  it  is  a  natural  con- 
sequence  of   certain  conditions. 

Penetration  and  resolving  power  are  antagonistic, 
or,  at  any  rate,  in  an  inverse  ratio,  and  can  only 
be  combined  to  a  certain  extent.  In  two  objec- 
tives of  the  same  power  and  angle,  one  cannot  have 
penetration  as  a  special  feature  and  the  other  re- 
solving power ;  they  will  be  almost  similar  in  these 
qualities,  provided  that  they  are  similarly  corrected. 
However,  if  they  are  not  similar  in  their  angular 
aperture,  the  one  of  narrow  angle  will  have  more 
penetration  than  the  other.  In  objectives  of  the 
same  angle  but  different  power,  the  one  of  low  power 
will  have  in  itself  more  penetration ;  it  will  be  simi- 
lar in  its  action  to  the  eye,  which,  when  an  object 
is  close  to  it,  can  distinguish  but  one  portion  of  it 
distinctly,  while,  as  its  distance  to  the  eye  is  in- 
creased, can  distinguish  various  parts  of  it  lying  at 
different  distances,  and  will  finally  see  other  objects 
outside  of  it.  By  looking  at  an  object  at  5  feet 
distance,  only  this  can  be  seen  plainly;  but,  at  10 
feet,  others  quite  a  distance  in  front  or  back  of  it 
can  be  seen    as  well. 


.  28 

Workinj*--Distance.— This  term,  strictly  consider- 
ed, is  an  invariable  quality  of  the  objective,  and  is 
the  distance  between  the  front  lens  in  the  objective 
and  an  uncovered  object,  when  the  objective  is  in 
focus  and  is  corrected  for  that  object.  All  objec- 
tives require  a  certain  amount  of  projecting  metal 
to  protect  the  front  lens,  and  this  with  a  certain 
thickness  of  the  cover-glass  lessens  it.  In  objectives 
with  fixed  mountings  this  may  be,  and  with  thick 
cover-glasses  is  considerable.  As  it  is  comparatively 
unimportant,  however,  for  the  working  microscopist 
to  know  the  working  distance  per  se  of  his  objectives, 
but  of  considerable  moment  to  know  what  the  actual 
space  between  the  objective  and  cover-glass  is,  it 
would  be  well,  in  the  mind  of  the  writer,  to  express 
it  as  available  working  distance. 

In  objectives  of  low  and  medium  power,  it  is  of 
little  consideration ;  but  where  it  must  be  expressed 
in  i-ioo  or  i-i,ooo  inch,  it  becomes  a  matter  of 
importance. 

Working-distance  is  spoken  of  as  being  long  or 
shorty  and  varies  not  so  much  with  the  power  as 
with  the  angular  aperture  ;  generally  the  working- 
distance  decreases  with  the  increase  in  angular  aper- 
ture, and  becomes  greater  as  the  angle  becomes 
narrower ;  it  was  for  a  long  time  considered  that 
these  two  properties  varied  according  to  a  fixed  rule, 
but  this  at  the  present  time  is  not  considered  to  be 
the  case.  While  in  objectives  of  the  same  angular 
aperture    it    may  vary    considerably,    it    may    in  others 


29 


of  difterent  angles  be  so  that  the  one  of  wider 
angle  may  have  the  greatest  working  distance.  The 
skill  of  the  optician  must  iu  a  considerable  manner 
determine  the    amount   of  it. 

It  will  be  seen  from  the  above  that  working-dis 
tance  stands  in  no  direct  relation  to  the  focal  dis- 
tance of  the  objective,  neither  to  its  nomenclature 
or  rating,  and,  it  may  be  added,  that  it  is  never  as 
great  as  the  focal  distance  of  a  single  lens  of  the 
same  magnifying  power. 

As  may  be  imagined,  there  are  a  variety  of  opin- 
ions as  to  what  constitutes  long  or  short  working- 
distance  in  a  certain  objective.  No  definite  rule  can 
be  laid  down  for  this,  as  it  is  conditioned  by  the 
skill  and  requirements  of  the  manipulator.  Although 
it  is  an  important  factor,  the  idea  that  it  should 
in  all  cases  be  as  great  as  possible,  is  erroneous, 
for,  while  it  may  be  true  in  a  dry  objective,  it  may 
be  the  cause  of  annoyance  in  one  with  immersion. 
On  several  occasions  it  occurred  in  the  experience  of 
the  writer  that  after  an  objective  had  been  com- 
pleted, it  was  found  that  its  working  distance  was  so 
large  that  the  immersion  fluid  would  run  out  from 
between  the  objective, and  cover-glass  when  the  in- 
strument was  inclined,  and  it  was  necessary  to  change 
the  objective  with  a  view  to  decreasing  its  working 
distance,    in    order    to    allow  its  convenient  use. 

Mag-nifyiiig-  Power.— This  is  a  question  of  vital 
importance  in  a  microsco{)e,  not  so  much  as  a 
quality    for  itself,   as   in  connection   with  the  resolving 


30 


power.  The  inquiry  should  not  be  simply  how  many 
diameters  an  instrument  will  magnify,  but  what  the 
precision  and  extent  of  its  definitions  are  under  a 
certain  magnifying  power.  If  a  high  magnifying 
power  is  all  that  is  desired,  this  may  be  obtained  to 
an  almost  unlimited  extent  by  means  of  simple  lenses 
which  may  be  procured  at  a  small  pecuniary  outlay ; 
but  these  do  not  give  a  distinct  image  nor  do  they 
make  structure  visible,  which,  be  it  remembered,  is 
the    purpose    of  the   microscope    to    do. 

The  normal  eye  can  distinguish  from  200  to  250 
lines  to  the  inch,  and  in  a  microscope  such  magni- 
fying power  should  be  used,  which  will  apparently 
bring  the  structure  which  is  sought  after  at  least  up 
^o  this  figure.  In  illustration,  take  a  \  inch  objec- 
tive of  98*^  and  a  \\  inch  eye  piece.  An  objective 
of  this  kind,  properly  corrected,  resolves  pleu?'Osig)na 
angulatuni^  in  which  the  average  lines  are  60,000  to 
the  inch.  With  the  above  eye-piece  it  is  utterly 
impossible  to  see  them,  while  if  it  is  re-placed  by 
a  1^  inch  or  \  inch,  they  can  easily  be  distinguished. 
This  is  not  owing  to  any  peculiar  quality  of  the  eye- 
piece, but  merely  to  the  fact  that  by  increasing 
the  magnifying  power,  the  dimensions  of  the  object 
have  been  increased  to  such  an  extent  that  the 
lines  have  apparently  been  separated  and  become 
visible  to  the  eye. 

Beginners  as  a  rule  are  apt  to  use  too  much 
magnifying    power   or  amplification^  and  often  attempt 


31 


to  view  a  large  surface  with  an  objective  which 
will  show  but  a  small  part  of  it.  It  must  not  be 
forgotten  that  the  apparent  field  of  view  is  decreased 
as  higher  powers  are  used,  and  that  a  low  power 
will  give  a  better  impression  of  a  large,  coarse  ob- 
ject and  its  relative  parts,  not  only  because  it 
makes  a  larger  surface  visible,  but  because  it  has 
more    penetration. 

In  objectives  of  the  same  power,  but  of  different 
angular  aperture,  the  magnifying  power  and  field  will 
always  be    the   same. 

The  following  table,  which  has  been  compiled,  will 
probably  be  of  assistance  to  the  beginner.  After  he 
has  become  better  acquainted  with  his  instrument 
his   judgment  will    dictate    to    him   what   to  do. 

A  power  of  25  diameters  will  show  a  surface  of 
about    1-5    inch    diameter. 

A  power  of  50  diameters  will  show  a  surface  of 
about   I- TO    inch   diameter.  * 

A  power  of  100  diameters  will  show  a  surface  of 
about    1-20   inch   diameter. 

A  power  of  500  diameters  will  show  a  surface  of 
about   i-ioo  inch  diameter. 

A  power  of  1,000  diameters  will  show  a  surface  of 
about  1-200  inch  diameter. 

This  table  is  approximately  correct  with  a  Huy- 
ghenian  eye  piece ;  with  a  Periscopic  almost  double 
the    amount    of  such  surface    will    be    shown. 


32 


Magnifying  power  may  be  obtained  by  tlie  eye- 
piece or  objective,  and  the  desirability  of  using  one 
or  the  other  for  this  purpose  was  for  many  years 
a   matter   of  spirited   discussion. 

Objectives  of  the  same  angular  aperture,  but  of 
different  powder,  will  give  identical  results,  by  bring- 
ing them  up  to  the  same  magnifying  power,  unless 
the  difference  is  considerable.  In  both  objectives 
and  eye  piece?  the  lenses  decrease  in  size  with  the 
increase  in  power  and  consequently  give  less  light ; 
and  while  this  one  objection  exists  in  the  objective 
an  additional  one  occurs  in  the  eye-piece,  in  that 
the  eye  must  be  brought  closer  to  the  eye-lens 
and  must  be  kept  more  strictly  in  the  optical 
axis,  which  at  a  long  sittmg  becomes  fatiguing. 
By  almost  common  consent  a  higher  power  than  ^ 
inch  eye-piece  should  not  be  used  in  prolonged 
work,  and  between  this  and  the  lowest  it  becomes 
a  matter  of  individual  choice.  All  responsible 
manufacturers  and  dealers  make  up  such  outfits  of 
stands,  objectives  and  eye-pieces,  which  experience 
has    shown    are    most    generally    useful. 

It  is  a  safe  rule  to  follow  in  all  work  on  rec- 
ognized forms  (objects  of  which  the  structure  is 
known)  not  to  use  a  higher  power  than  is  necessary 
to  properly  study   them. 

Selecting-  Objectives.— It  is  supposed  that  what 
has  been  stated  regarding  the  properties  of  an  ob- 
jective,   has    been   sufficiently   expHcit    to    enable    the 


33 


beginner  to  make  a  suitable  selection,  when  he  finds 
that  other  objectives  than  those  which  he  possesses 
are  necessary.  Although  from  a  theoretical  stand- 
point it  is  true  that  objectives  are  optically  more 
perfect  as  they  increase  in  their  angular  apertures, 
it  is  not  by  any  means  said  that  this  is  a  rule  in 
practice.  A  great  portion  of  every-day  work  can 
be  done  with  less  loss  of  time,  with  more  comfort 
and  with  the  attainment  of  as  high  results  with 
objectives  of  medium  angular  aperture  and  magni 
fying  power.  A  1-5  inch  of  75°  or  110°  for  in- 
stance is  a  very  valuable  objective  in  any  outfit,  al- 
though its  performance  is  limited;  when  more  is  de- 
sired than  this  will  give,  recourse  must  be  had,  first 
to  objectives  with  wider  angular  aperture  (this  should 
be  considerable ) ;  second  to  those  with  more  mag- 
nifying   power. 

In  this  connection  it  is  considered  important  to 
mention  the  suggestions  of  Dr.  Geo.  E.  Blackham, 
who  from  his  knowledge  of  the  optical  principles 
involved  in  the  microscope,  and  from  his  long  ex- 
perience and  delicacy  in  manipulation,  is  pre-emi- 
nently  fitted    to    make    recomendations. 

His   ideal    series    is    as    follows : 

OBJECTIVES. 

One  4  inch  12°  air  angle  =  o.  10  numerical  ap- 
erture. 

One  I  inch  30°  air  angle  =  0.26  numerical  ap- 
erture. 


34 


One    1-6    inch    140°    air    angle    =  0.94    numerical 
aperture. 

One    1-8    homogeneous    immersion     1.42     numerical 
aperture. 

EYE-PIECES. 

One    2    inch    Huyghenian. 
One    I 


((  a 


One   }      " 


(( 


One   I      "      Sohd. 


The  4  inch  and  i  inch  objectives  are  of  course 
non  adjustable,  while  the  two  latter  have  cover  cor- 
rection. 

This  series  is  supposed  to  cover  every  direction 
of  investigation.  In  a  great  many  cases,  of  course, 
such  a  low  power  as  would  be  given  by  the  4  inch, 
would  be  useless;  in  the  above  table  the  highest 
power  is  obtained  by  the  use  of  a  ^  inch  objective 
and  ^  inch  eye-piece,  whereas  the  same  can  be  ac- 
complished by  a  1-12  inch  objective  and  J  inch 
eye-piece,  or  1-16  inch  objective  and  i  inch  eye- 
piece. As  stated  before,  this  becomes  a  matter  of 
individual  choice. 

Be  distrustful  of  all  objectives  in  which  the  chief 
inducement  is  a  very  low  price ;  they  are  almost 
invariably  useless  for  any  reliable  work.  Have  a 
distrust  especially  of  all  "nameless"  objectives.  It 
is   safe   to  assume   that   if  the  maker  can    not    attach 


35 

liis  name,  he  is  doubtful  of  their  superiority,  if  not 
convinced  of  their  inferiority.  Any  maker  of  re- 
sponsibility will  say  without  hesitation,  that  he  can 
produce  objectives  at  less  than  one-half  their  present 
cost,  if  he  had  the  assurance  that  they  would  be 
accepted  as  first  put  together,  as  the  cost  of  mere- 
ly making  and  mounting  lenses  is  considerable  less 
than  the  cost  of  making  proper  corrections.  In 
this  case  however  they  would  be  of  varying  and 
inferior   quality. 

In  purchasing  a  microscope  a  beginner  may  be 
easilv  misled  by  the  enticing  appearance  of  an 
object,  which  may  be  due  not  so  much  to  the  in- 
strument as  to  the  object  itself,  and  if  the  optical 
parts  are  inferior,  it  will  require  but  a  short  ex- 
perience to  become  convinced  of  it — usually  as  soon 
as  a  comparison  can  be  made  with  reliable  work. 
The  investment  in  one  of  these  objectives  is  not 
only  a  source  of  disappointment,  but  usually  proves 
to  be  a  pecuniary  loss,  as  it  is  usually  followed  by 
a   fresh   outlay   in   responsible   work. 

It  is  of  ordinary  occurrence  that  such  objectives 
as  have  just  been  spoken  of  are  sent  to  the 
writer's  firm  with  the  request  to  examine  them  and 
rectify  the  faults;  but  an  examination  almost  inva- 
riably proves  that  the  cost  of  doing  this  is  consid- 
erably greater  than  purchasing  a  new  objective  of 
the  same  power,  and  it  would  not  then  even  be 
equal   to   the   latter. 


36 


Eye-Piece— Huyg-lieuiaD.— This    is    now    in   gen- 
eral   use,    and    consists     of    two    plane-convex    lenses. 

It  receives  its  name  from  the 
inventor,  who  first  apphed  it  to 
the  telescope.  The  eye-lens  is 
the  small  lens  nearest  the  eye, 
5  and  the  field-lens  or  collective^ 
as  it  is  also  called,  is  the  large 
one  nearest  the  objective.  A 
diaphf'agm  is  placed  between 
them,  and  gives  a  sharply  de- 
fined field.  This  eye-piece  is 
Fig  „  also    called   negative.,   as    its   focal 

point  is  between  the  two  lenses  (at  the  diaphragm) 
in  contradistinction  to  a  positive.,  in  which  the  focal 
point  is  outside  of  and  below  the  field  lens. 


1 


Solid  Eye-Piece. — This  was  the  invention  of  the 
late  R.  B.  ToUes,  and  also  belongs  to  the  class  of 
negative  eye-pieces.  It  is  called  solid  from  the  fact 
that,  instead  of  being  composed  of 
two  lenses,  it  consists  of  one  piece 
of  glass,  which  is  cut  to  a  cyHn- 
drical  form,  and  on  the  ends  of 
which  the  proper  curvatures  are 
ground;  the  diaphragm  is  made  by 
cutting  a  circular  groove  into  the 
glass  at  the  proper  distance  between  the  two  sur- 
faces, which  is  then  filled  up  with  an  oi)aque 
pigment. 


1/ 


Fi 


g.    12. 


37 


These  eye-pieces  are  only  made  in  high  powers, 
as  optical  glass  is  usually  not  of  sufficient  homo 
genuity  to  make  low  powers,  and  their  cost  would  be 
too  considerable,  without  a  corresponding  advantage. 
For  high  powers  they  are  superior  to  the  Huyghen- 
ian,  in  that  they  give  a  better  illuminated  field,  as 
there  is  less  loss  of  light  by  absorption  through  the 
glass  than  by  refraction  at  the  two  additional  sur- 
faces of  the  eye -lens  and  field -lens  in  the  Huy- 
ghenian. 

Periscopic  Eye-Piece.— This  consists  of  a  triple 
eye-lens  and  single  field-lens.  Its  predominant  fea- 
ture is  a  very  large  and  flat 
field,  with  almost  all  objectives. 
In  this  respect  it  has  a  consider- 
able advantage  over  the  Huygh- 
D  enian  and  Solid.  It  is  positive 
and  therefore  well  adapted  for 
micrometer  work,  as  it  is  focused 
like  a  magnifier,  and  its  magni- 
fying power  remains  constant, 
while  with  the  Huyghenian  it  is 
variable,  from  the  fact  that  the 
eye-lens  alone  is  focused,  thus  varying  its  distance 
from  the  field-leus,  and  consequently  the  magnifying 
power. 

Nomenclature.— The  rating  of  eye  pieces  was 
formerly,  and  is  to  a  considerable  extent  to-day,  by 
letters.  This  method,  however,  is  arbitrary,  as  the 
letters  of    difterent    makers    have    a     totallv     different 


38 


significance,  so  that  nothing  Hke  a  standard  exists. 
This  fact  induced  the  American  Society  of  Micro- 
scopists  to  endeavor  to  estabHsh  a  universal  method, 
and  after  the  matter  had  been  given  careful  attention 
for  several  successive  years,  it  finally  adopted  the 
method,  which  rates  them  according  to  their  magnifying 
power,  the  same  as  that  which  has  been  used  in  ob- 
jectives. This  gives  an  approximate  idea  of  the 
magnifying  powers ;  thus,  an  eye  piece  marked  i  inch 
or  by  a  letter  signifying  the  same,  shows  that  it 
magnifies  about  lo  diameters;  one  of  ^  inch,  20  dia- 
meters, and  so  on. 

Flatness  of  Field.— Although  this  depends  mainly 
upon  the  objective,  the  absence  of  it  may  be  owing 
to  a  faulty  construction  of  the  eye-piece.  If  it  is  so 
prominent  as  to  be  easily  noticeable,  and  to  the  same 
degree  with  a  number  of  objectives,  it  may  be 
ascribed  to  the  eye-piece.  It  must,  however,  be  re- 
membered that  an  absolutely  flat  field  has  not  yet 
been  obtained ;  it  may  be  closely  approached  by  de- 
creasing the  diameter  of  field  to  less  than  its  normal 
size. 

Size  of  Field.— Quite  a  general  but  erroneous 
idea  prevails  that  the  size  of  the  tube  has  an  influ- 
ence on  the  size  of  the  field.  Except  in  eye-pieces 
of  very  low  power,  or  with  tubes  of  smaller  than 
usual  dimensions,  this  is  not  so.  It  must  be  remem- 
bered that  a  Huyghenian  eye-piece  admits  of  a  definite 
size  of  field,  and   this    is    regulated  by  the  opening  in 


39 


tlie  (liapliragm  ;  the  same  size  of  oi)ening  is  usctl  in 
all  of  the  same  power,  whether  it  is  for  an  eye-i)iece 
of  a  large  or  a  small  diameter. 

A  misconception  also  exists  as  to  the  definition 
of  field.  Such  inquiries  are  often  made  as:  "as  we 
understand  it,  a  wide-angle  objective  gives  a  larger 
field";  but  it  does  nothing  of  the  kind.  The  angu- 
lar aperture  has  no  bearing  whatever  on  the  size  of 
the  field.  The  field  of  vieiv^  or  that  which  is  shown 
of  the  object's  surface,  is  determined  by  the  i:)Ower  of 
the  objective  and  eye-piece. 


REQUISITES    FOR    WORK. 


The  necessary  material  for  preparing  and  mounting 
specimens  will  be  found  enumerated  in  books  which 
are  devoted  to  this  purpose.  It  is  the  intention  to 
make  such  recommendations  in  this  chapter  which, 
if  not  absolutely  necessary,  will  be  found  convenient 
and    will    aid   in  facilitating  work. 

One  of  the  first  requisites  for  the  proper  use  of 
the  microscope  is  a  thorough  knowledge  of  its  parts 
and  an  acquaintance  with  the  optical  principles  in- 
volved. For  this  purpose  the  writer  earnestly  requests 
a  perusal  of  the  preceding  pages,  and  is  convinced 
that  in  cases  where  no  previous  knowledge  of  the 
mstrument  has  existed,  work  will  be  done  with  far 
more  ease,  in  much  less  time,  and  with  a  greater  de- 
gree of  satisfaction.  Ignorance  of  the  instrument's 
capacity  may  lead  to  an  idea  that  it  is  inferior  and 
thus  be  the  means  of  its  final  abandonment ;  and  in 
place  of  the  anticipated  pleasure  there  may  arise  a 
feeling  of  bitterness  and  disappointment  for  all  future 
with  everything  connected  with  it.  There  are  innumer- 
able cases  of  this  kind  and  they  have  induced  a  belief 
that  it  is  difficult  to  acquire  a  practical  manipulation 
of  the  microscope,  whereas  such  is  not  the  case 
when  a  limited  time,  properly  applied,  is  devoted  to  it. 


41 


Workiiif*-  Tablo.— A  firm  tal)lc  sliould  be  used, 
preferably  one  with  three  legs,  as  this  will  always 
be  firm,  no  matter  how  uneven  the  lioor  is,  and  if 
it  can  be  arranged,  should  be  devoted  to  this  pur- 
pose only.  One  with  a  round  or  square  top  of 
three  feet  provides  ample  room.  Although  not  neces- 
sary, a  table  with  a  revolving  top,  provided  with 
clamp,  is  very  convenient,  as  with  this  two  or  more 
persons  may  make  observations  without  changing 
their    seats. 

A  very  neat  arrangement  for  a  table- top  is  that 
suggested  and  used  by  Dr.  J.  E.  Reeves.  He  places 
upon  an  ordinary  table  three  or  four  thicknesses  of 
white  paper  and  upon  these,  a  plate  of  polished 
glass,  as  large  as  the  top ;  this  can  be  procured  of 
almost  any  glazier  at  a  low  price.  It  is  pleasant 
to    work    upon    and    will    not  soil. 

As  in  almost  all  cities  there  is  more  or  less  continual 
vibration  from  wagons  upon  the  paved  streets,  the 
writer  suggests  an  eft'ectual  remedy.  Take  a  thin  board, 
say  half  an  inch  thick,  of  a  sufficient  size  to  receive  the 
microscope ;  fasten  on  the  upper  side  at  two  opposite 
ends,  cleats  of  about  i  inch  square  and  countersink 
into  these  through  the  board  four  spiral  springs  of 
such  tension  that  when  they  bear  the  weight  of  the 
instrument,  the  bottom  of  the  board  will  be  about 
^  inch  from  the  table. 

Have  the  working-table  provided  with  drawers  and 
arrange  receptacles  for  the  accessories,  secure  from 
dust,    but    at     a    convenient    point    to    reach.       When 


42 


tlie  instrument  is  not  in  use,  put  it  into  its  case  or 
cover  it  in  a  manner,  so  that  it  shall  be  free  from 
dust.     For  this    purpose  a  large  bell  glass  is  best. 

Koom— If  possible  a  room  should  be  selected 
facing  the  north,  as  the  light  in  this  direction  is 
most  constant.  It  will  prove  a  great  saving  of  time 
if  all  or  a  portion  of  it  can  be  permanently  ar- 
ranged to  receive  the  entire  working  outfit.  It 
should  also  be  chosen  with  a  view  to  its  being 
free   from    disturbance. 

Lig-lit.  As  stated,  the  light  from  the  northern 
sky  is  most  desirable,  and  that  from  a  white  cloud 
is  preferable  to  that  from  a  blue  sky.  On  account 
of  its  intensity,  direct  sunlight  should  never  be  used; 
but  if  modified  by  a  white  curtain  or  reflected  from 
a    white    wall   it   is    excellent. 

For  lamp  light  an  ordinary  flat  wick  kerosene  or 
student  lamp  is  well  adapted.  The  Hitchcock  lamp, 
from  its  better  combustion  is  still  better,  as  its 
color  more  nearly  approaches  white.  The  ideal 
artificial  light  is  that  from  an  electric  light.  Gas 
light  is  not  desirable  as  it  is  seldom  sufticiently 
steady. 

Position  of  Lig-lit.— The  relation  of  the  micro- 
scope to  the  source  of  light  is  principally  a  matter 
of  personal  convenience.  With  daylight  it  makes 
little  difference  whether  it  is  at  the  front  or  side 
of  the  instiument,  although  the  writer  prefers  it  at 
the   front,    as    the    manipulation    of    the    object    does 


43 


not  obstruct  it ;  but  the  lamp  should  be  placed  at 
the  right  or  left  side,  within  easy  reach  of  the 
hand  for  the  purpose  of  controlling  it.  The  writer 
suggests  that  the  beginner  make  it  a  habit  at  the 
outset  to  place  it  on  the  side  of  the  instrument  oppo- 
site to  the  unoccupied  eye,  as  the  tube  then  places 
the   latter    in  the  shadow. 

Which  Eye  to  Use. — In  a  binocular  instrument 
both  eyes  are  used,  but  in  a  monocular  only  one 
is  used,  and  it  depends  upon  a  trial  which  is  best 
suited.  A  large  proportion  of  persons  are  afflicted 
with  astigmatism,  often  without  knowing  it,  and 
when  this  exists  it  may  be  in  one  eye  or  when  in 
both,  may  be  to  a  greater  extent  in  one  than  in 
the  other.  Its  presence  may  prevent  the  eye  from 
observing  fine  detail ;  but  whichever  eye  is  found  to 
be  best  suited  should  be  used.  When  both  eyes 
are  alike,  it  is  sometimes  advisable  to  change 
from    one    to    the    other. 

It  should  be  made  a  habit  at  the  outset,  and 
strictly  adhered  to,  to  keep  both  eyes  open.  A  little 
difficulty  may  be  found  to  do  this,  as  the  eye 
which  is  free  will  probably  observe  the  objects  up- 
on the  table ;  but  as  soon  as  the  mind  becomes 
fixed  upon  what  it  sees  in  the  microscope,  this 
impression  disappears.  After  a  time  it  will  be  found 
to  require  no  exertion  and  will  certainly  add  to 
the  ease  and  comfort  of  the  mani})ulator  while 
working. 


44 


The   Ward  Bye  Shade,  Fig.    14,    will    prove    of 

assistance  in  ac- 
quiring the  above 
mentioned  habit, 
and  besides  this, 
etfectually  excludes 
the  light  from  the  eyes.  It  is  made  of  hard  rub- 
ber and   is    attached    to    the  tube   of   the    microscope. 

Order.— Among  the  requisites  for  successfully 
[prosecuting  work  with  the  microscope  are  a  strict 
observance  of  the  instructions,  even  if  they  appear 
superfluous,  a  systematic  way  of  doing  work,  and 
cleanliness.  Have  a  place  for  every  article  which 
is  required,  so  that  the  hand  may  immediately  be 
placed  upon  it ;  after  it  has  been  used,  clean  it 
before  putting  it  aside ;  keep  strange  hands  from 
your  apparatus,  unless  you  are  assured  that  a 
knowledge    of  its    manipulation    exists. 


HOW    TO     WORK. 


To'  Set  up  the  i  list  ruiiieat— Draw  the  instru- 
ment from  the  case  by  grasping  the  base,  and  free 
it  from  dust.  If  it  has  draiv  tube,  bring  it  to  its 
standard  length,  which  is  indicated  by  a  ring,  by 
as  httle  of  a  screw  motion  as  possible ;  if  the  draw 
tube  is  highly  polished,  its  surface  will  be  best 
retained  by  observing  the  above  precaution.  See 
that  the  mirror  and  largest  aperture  of  the  dia- 
phragin  are  in  a  central  position — in  the  optical  axis. 
After  being  convinced  that  the  eye-pieces  are  clean, 
place  one  into  the  tube ;  then  remove  the  objectives 
from  their  cases  and  after  first  having  increased 
the  distance  between  the  stage  and  tube  by  means 
of  the  mi/led  heads  of  the  pinion^  attach  the  lowest 
power  to  the  nose-piece,  by  using  both  hands,  being 
careful  that  it  is  as  near  as  possible  in  the  optical 
axis  while  screwing  it  on.  Then  incline  the  body 
by  placing  the  left  hand  upon  the  base  and  draw- 
ing with  the  right  hand  upon  the  arm ;  be  careful 
not  to  pull  on  the  tube,  as  it  may  prove  too 
heavy  a  strain  upon  this  or  the  fittings.  Incline 
the  body  of  the  microscope  until  the  eye-piece  about 
reaches  the  level  of  the  eye,  so  that  when  an 
observation    is    made    the    position     is    as    comfortable 


46 


as  possible  ;  the  neck  should  not  be  strained, 
neither  should  the  chest  be  compressed.  Next 
place  the  slide  with  a  transparent  object  upon  the 
stage^  by  sliding  it  under  the  spring  clips  and  get 
it  as  near  as  possible  in  the  center  of  the  open- 
ing; for  an  object  anything  near  at  hand,  such  as 
a  piece  of  printed  paper  or  cotton  fibres  will  do. 
Watching  the  slide,  adjust  the  mirror  until  it  is 
seen  that  the  light  strikes  the  object ;  incline  the 
head  to  the  level  of  the  stage,  and  observing  the 
objective,  rack  it  down  to  within  \  inch  of  the 
object.  Again  placing  the  eye  at  the  eye-piece, 
reverse  the  motion  of  the  milled  heads  and  ob- 
serving the  field  continue  the  upward  motion  of  the 
body    until  the    image  of  the    object  appears  in   view. 

Centering-  Stage.— If  the  microscope  has  a  re- 
volving stage,  turn  this  to  see  whether  the  object 
or  portion  of  it  lying  in  the  center  of  the  field, 
remains  in  the  optical  axis.  It  was  true  when  the 
instrument  was  shipped,  but  may  have  changed  during 
transportation.  If  not  centered,  loosen  the  screw 
holding  it  to  the  arm,  by  means  of  the  steel  pin,  just 
sufficiently  that  by  the  exertion  of  a  Httle  pressure  it 
can  be  moved.  After  having  observed  first  which 
portion  of  the  object  remains  stationary  during  its 
revolution  (this  evidently  is  its  center)  move  the 
stage  so  that  this  point  will  be  in  the  center  of 
the  field,  and  then  tighten  the  screw.  If  the  point 
lies  outside  of  the  limits  of  the  field,  its  direction 
can     be    noted    and    the    stage     moved     accordingly. 


47 


Wliere   centering  screws   are  provided    in    tlie    instru- 
ment,   this   is    a   simple    matter. 

If  the  coarse  adjustment  does  not  prove  sensitive 
enough  to  focus  easily,  adjust  by  the  fine  adjust- 
ment by  taking  the  head  of  the  micrometer  screw 
between  the  thumb  and  first  finger  and  move 
toward  the  right    or   left    as    may  be    necessary. 

It  may  here  be  said  in  passing  tliat  the  rack 
and  pinion  should  be  so  well  fitted  that  they  should 
permit  the  adjustment  of  low  power  objectives  with 
the  greatest  ease,  and  should  work  without  the 
slightest  lost  motion  with  a  1-5  or  1-8  inch  objec- 
tive. This  point  is  the  criterion  of  workmanship 
in  an  instrument,  and  if  it  is  found  to  have  the 
least  back-lash^  or  is  not  perfectly  smooth,  it  may 
safely  be  assumed  that  the  instrument  is  of  inferior 
workmanship. 

If  the  fine  adjustment  does  not  act,  the  screw 
has  either  come  to  its  stop  or  has  "run  out,"  and 
must  be  brought  into  action  again ;  the  range  of 
movement  in  almost  all  fine  adjustments  is  quite 
short  and  constant  care  must  be  taken  to  keep  it 
at  about  a  medium  point.  If  the  object  is  found 
not  to  give  a  full  view  or  is  not  in  the  center  of 
the  field,  it  must  be  moved  on  the  stage,  but  it 
must  be  remembered  that  a  movement  in  one  di- 
rection causes  an  apparent  opposite  movement  in 
the  field.  At  first  this  movement  will  be  in  jerks, 
but  after  a  little  practice  the  necessary  sensitiveness 
of    touch    is    acquired     to    give    it    more    steadiness. 


48 


From  tlie  fact  that  the  bottom  of  tlie  glass  shde 
offers  a  large  frictional  surface,  absolute  dependence 
can  never  be  placed  upon  the  movement ;  especial- 
Iv  in  medium  and  high  powers  will  it  be  found 
that  a  specimen  or  portion  of  one  will  be  lost  to 
view  in  this  manner,  and  that  considerable  time  is 
<?s:^^  ______  consumed     in     finding 


it    again.       A     simple 
device   for  overcoming 


^^fi^^^^^gg.  ^~~        ^^''^^'     ^^"'^^     *^^^    which 
^'"^411^^  insures    a    high   degree 

tig-  15-  of   comfort,   is   a  glass 

stage    and    slide  carrier^    Fig.     15. 

It  should  be  made  a  rule  at  the  outset  to  use 
one  hand  for  the  stage  and  the  other  for  the  fine 
adjustment. 

Illuniinatioii.— It  should  now  be  observed  wheth- 
er the  field  is  equally  illuminated.  Too  much  stress 
can  not  be  laid  on  this  point,  as  it  is  one  which 
IS  easily  overlooked  and  is  often  the  cause  of  con- 
siderable mischief  If  the  light  comes  through  a 
window  a  well  defined  image  of  the  sash  is  reflect- 
ed by  the  mirror,  and  with  a  low  power  objective 
this  can  easily  be  seen ;  unless  the  mirror  is  cor- 
rectly adjusted  the  field  will  appear  to  be  crossed 
by  dark  bands.  In  the  case  of  lamp  light  the 
flame  is  reflected  and  has  a  similar  eftect.  With 
high  powers  this  fault  is  not  so  easily  noticed,  and 
for  this  reason  require  the  more  care ;  proper  res- 
olution   may   in    this    manner    be    partially   or   totally 


^9 


destroyed.  The  remedy  is  either  by  shifting  the 
mirror  or  by  varying  its  distance  from  the  object. 

Attacliiiag"     High-Power    Objectives.— As     the 

difficulty  of  properly  getting-  an  object  or  a  certain 
portion  of  it  in  the  field,  increases  with  the  mag- 
nifying power,  it  is  a  good  rule  to  use  the  lower 
power  objective  as  a  "finder;"  after  getting  the 
point  to  be  farther  examined  in  the  center  of  the 
field,  remove  the  objective  and  attach  the  higher 
power,  and  after  following  the  procedure  of  focus- 
ing as  with  the  low  power,  except  that  the  object- 
ive should  be  brought  ahiiost  in  contact  with  the 
cover,  tlie  point  will  be  seen  in  the  field  or  will 
be   found    to   be    close    to    it. 

This  plan  of  focusing  as  suggested  above  is  al- 
ways a  good  one  to  follow  and  is  observed  by 
many  of  the  most  expert  manipulators.  In  many 
cases  however  the  focus  is  obtained  Avithout  this  pre- 
caution, by  watching  the  field  as  the  objective  is 
brought  down  toward  the  object,  but  is  often  fol- 
lowed by  disastrous  results.  Almost  any  person 
who  has  used  the  microscope  for  any  length  of 
time  is  without  question  aware  that  valuable  prepa- 
rations have    been    destroyed    in    this    way. 

Double  Nose-Pie6e.— A  very  valuable  accessory 
is  in  use  called  the  double  nose-piece,  (Fig.  i6,) 
which  obviates  the  changing  of  objectives  and  it  is 
safe  to  say  that  those  who  have  once  used  it 
would  not    be  without   it.     It  receives  two  objectives 


50 


Fig.    i6. 

and  is  attached  to  the  microscope  in  the  same 
manner  as  an  objective.  In  those  made  by  the 
Bausch  &  Lomb  Optical  Co.,  their  3-4  inch  and 
1-5  inch,  I  inch  and  1-4  inch  objectives,  student 
series  are  fitted  in  pairs  so  that  they  correspond 
in  focus  without  adjusting  and  are  exactly  centered. 
Each  end  of  the  nose-piece  is  marked  for  the 
proper  objective. 

How  to  Work.— It  is"  now  supposed  that  the  in- 
strument is  ready  for  work.  To  start,  it  is  well  for 
the  beginner  to  provide  a  few  prepared  specimens, 
as  these  will  help  him  considerably,  if  it  is  his  in- 
tention, as  it  should  be,  to  prepare  them  later  him- 
self. Whatever  branch  of  study  he  is  going  to  fol- 
low, a  slide  of  Pleiirosignia  a?igulatum,  dry,  will  be 
valuable  to  practice  upon  ^nd  to  determine  the 
quahty  of  his  higher  power  objective.  In  this  latter 
respect,  however,  the  writer  would  advise  the  begin- 
ner to  guard  against  expressing  an  opinion  too  soon. 
He  knows  of  many  cases  where  the  optician's 
claims   were   flatly   denied,    when    often    a    few   words 


51 


of  advice  by  letter  or  a  few  minutes  of  manipulation 
would  resolve  the  diatom,  when  it  had  not  been  done 
by  the  beginner,  and  would  do  it  so  easily,  that  it  be- 
came a  wonder  how  it  could  be  avoided  in  any 
hands. 

For  a  low-power  objective,  the  proboscis  of  a  blow- 
fly is  probably  the  most  suitable  and  at  the  same 
time,  most  interesting  object.  Place  this  upon  the 
stage,  and,  after  getting  it  as  close  as  possible  to 
the  center  of  the  opening  in  it,  focus  by  means 
of  the  coarse  adjustment.  If  only  a  portion 
of  it  can  be  seen  and  if  it  is  desired  to 
see  a  larger  surface,  the  length  of  tube  may  be 
contracted  by  means  of  the  draw-tube.  In  this  case 
the  object  will  be  placed  out  of  focus^  and  another 
adjustment  becomes  necessary.  If  a  higher  power  is 
desired,  the  draw-tube  may  be  extended.  Observe 
whether  the  field  is  well  illuminated,  and  if  not, 
bear  in  mind  what  has  been  said  on  this  subject. 
If  the  object  appears  milky  or  the  light  is  so  in- 
tense as  to  be  painful  to  the  eye,  which  is  of  usual 
occurrence  to  the  beginner,  the  diaphragm  should  be 
turned  from  one  aperture  to  another  until  a  marked 
difference  is  seen ;  or,  the  plane  mirror  should  be 
used.  In  this  connection,  it  is  well  to  state  that 
the  above  precautions  should  always  be  observed 
with  low  powers,  unless  the  object  is  thick.  Now 
use  the  micrometer  screw  and  note  carefully  the 
beautiful    structure   which    is  opened   to    view.     After 


52 

sufficient  time  has  been  spent   upon     this,    the    objec- 
tive    may     be    replaced     by    a  higher    power  and  the 
object    by    a    shde    of    P.    a7igidatum ;    focus    upon 
this,     being      mindful    of     the    suggestions    previously 
given,     and     do    not    fail  to    observe    what    has     been 
said      in     regard    to     a     well    illuminated     field.       If 
lamp-light    from    a    flat    wick    is    used,    turn    the  edge 
of  the    flame    toward    the    mirror,    and    use    the    con- 
cave   side  of  the    latter.     If   the    diaphragm    is    in    an 
adjustable    sub  stage,   bring    it    as    close   to   the  stage 
as    possible,    or,    whether    here     or    attached    to    the 
stage,   it   may   as   well    be    removed    for   the    present. 
Observe    now  whether  outside   of  the    central  rib   any 
hnes    can    be    seen   upon    the  surface   of  the  diatoms ; 
if   not,    vary    the     distance    of    the     mirror     from    the 
object;    or,    if    lamp-light    is    used,     bring    the    lamp 
closer   to    or   remove    it    from    the    instrument    in  one 
Hne,    so    that    the    illumination  will   not   disappear.     If 
this    does  not  bring  out  the  lines  swing  the    mirror-bar 
from    the     central    position     into    an    oblique   one,  on 
the    side   opposite    to    that    of  the    light    and    readjust 
the    mirror;    in     doing     this    grasp    the    ends    of   the 
mirror-fork   between    the     thumb     and    middle    finger 
and  move   the  mirror  by  the  first  finger.     If  the  field 
can    not    be    evenly   illuminated,    it     is    evidence    that 
the   mirror   is    beyond    the    limit    of    angular    aperture 
in  the  objective,  and  it  must  therefore  be  brought  back 
until   it   is.     It    must  here  also  be  noticed   that  if  the 
diaphragm    is    still    attached    to    the     instrument    and 
does    not    swing    with    the    mirror,  it    may  also  be  the 


means  of  cutting  ofl"  light.  By  means  of  the  micro- 
meter screw  carry  the  fine  adjustment  back  and 
forth  beyond  the  plane  of  the  object  and  observe 
closely  whether  any  lines  can  be  distinguished.  It 
is  very  probable  that  they  will  show  ;  but  if  not,  the 
ca^use  should  be  determined.  It  may  be  that  the 
magnifying  power  is  not  sufficiently  great,  and  in 
this  case  a  higher  power  eye-piece  should  be  used, 
or  the  cover  glass  may  be  more  or  less  than  the 
normal  thickness,  which  would  cause  a  spherical 
over  or  under-correction  in  the  objective.  In  this 
case  the  lines  would  appear  when  the  diatom  is  not 
in  focus.  If  the  objective  is  a  non-adjustable  one, 
the  proper  correction  may  be  approximately  reached 
by  means  of  the  draw-tube.  If  the  lines  appear  over 
the  plane  of  the  object,  it  shows  over-correction, 
and  the  length  of  tube  should  then  be  decreased, 
and  contrary  when  the  lines  show  below  or  beyond 
the  plane  of  the  object.  If  the  above  directions 
have  been  followed,  the  lines  cannot  fail  to  be  seen 
with  a  moderately  good  1-4  or  1-5  inch  objective  ; 
but  if  they  are  not,  the  trial  should  be  repeated. 
Again,  be  careful  to  have  no  obstruction  between  the 
course  of  rays  from  the  mirror  to  the  stage ;  get 
good  illumination  on  the  object ;  observe  well ;  keep 
the  instrument  in  such  a  position  that  the  object  is 
not  illuminated  from  anv  other  direction  than  from 
the    mirror. 

When    the    diatoms    are    7-esolved    in    this    manner, 
the  lines  will    appear  to   be  diagonal  in    some ;  lojigi- 


54 


tudijtal  or  transverse  in  others,  according  to  their 
position ;  and,  if  the  resolution  is  very  good,  these 
lines  will  furtlier  resolve  themselves  in  minute  beads 
of  a  hexagonal  form. 

It  will  now  be  well  to  bring  the  mirror  more 
nearly  to  a  central  position ;  do  this  at  intervals  of 
about  lo  degrees,  and  note  the  appearance  at  each 
decrease  of  obliquity.  It  will  be  found  that  as  the 
mirror  approaches  the  optical  axis  the  lines 
will  appear  to  become  more  faint,  and  may 
disappear  before  central  illumination  is  reached  ;  in 
this  case  it  will  be  well  to  begin  again.  An  endea- 
vor should  be  made  to  make  each  attempt  give 
better  results  than  the  preceding  one.  Repeated 
trials  will  not  only  impress  the  various  phenomena 
upon  the  mind,  but  will  cause  a  notable  improve- 
ment m  manipulative  skill,  and  thus  a  better  per- 
formance   in    the    objective. 

Until  now  we  have  assumed  that  transyjiitted  light 
has  been  used.  We  will  now  suppose  that  the  ob- 
ject is  not  sufficiently  transparent  to  use  this  method; 
the  object  is  then  said  to  be.  opaque^  and  requires  a 
different  procedure.  We  will  say  that  i^  is  desired 
to  examine  an  insect ;  it  may  be  attached  to  a 
slide,  or,  what  is  better  still,  may  be  fastened  in  a 
stage  forceps — (Fig.  17), — as  it  may  then  be  turned 
and  viewed  from  all  sides.  The  low  power  objec- 
tive should  again  be  attached ;  after  having  been 
focused,    it    will    be    found    that    the    light    is    insuf- 


55 


ficient  to   illumi- 
nate    it.        The 
mirror  bar  should 
^^^s-  17-  now     be     swung 

upon  its  axis  around  the  stage  to  a  point  above  it, 
so  it  will  be  at  an  angle  of  about  45  degrees  to  its 
surface.  If  a  lamp  is  used  and  is  in  the  same  posi- 
tion as  when  used  with  transmitted -light,  it  is  probable 
that  the  tube  of  the  instrument  will  obstruct  the  light, 
and  it  is  then  well  to  move  it  toward  the  front.  Using 
the  concave  mirror,  adjust  it  so  that  the  light  will 
be  concentrated  upon  the  object,  by  watching  it 
directly,  and  then  observe  through  the  tube.  If  it 
is  not  sufficiently  illuminated,  continue  to  adjust  the 
mirror ;  also  vary  its  distance  from  the  object  and 
swing  the  mirror-bar  to  a  higher  or  lower  point.  It 
often  occurs  that,  under  the  best  conditions,  the 
need  of  better  illumination  is  felt ;  in  this  case  a 
bulls-eye  condenser — (Fig.  18) — should  be  procured. 
It  will  be  found  that  this  will  become  a  useful  and 
perhaps  necessary  accessory  in  work  outside  of  this. 
Place  it  close  to  the  instrument  and  set  the  bulls- 
eye  between  the  object  and  source  of  light,  with 
the  plane  side  toward  the  object ;  if  an  ordinary 
hand  lamp  is  used,  it  will  be  necessary  to  elevate 
this  to  about  the  height  of  the  eye -piece,  and  if  it 
is  to  be  used  often  in  this  position,  a  special  sup- 
port should  be  made  for  this  purpose. 


56 


Fig.    1 8. 

I^ow  power  objectives  are  usually  used  on  opaque 
objects,  but  sometimes  a  higher  power  is  desired. 
Unless  one  is  constructed  with  a  view  to  opaque  illumi- 
nation its  working  distance  is  usually  so  short  that  it  will 
prevent  the  light  from  striking  the  object.  A  1-4  or 
1-5  objective,  of  75  degrees,  has  sufficient  working- 
distance,  and  its  mounting  is  made  conical  in  the 
front,  so   that   it   will   allow    it. 


57 


.-^'Ch:.. 


Dark  Ground  IlluiiiiuatioD.— This  method  is 
not  in  general  use,  probably  because  it  requires  a 
special  accessory,  although  it  yields  beautiful  effects. 
It  is   accomplished    by   means    of  a   paraboloid — (Fig. 

19) — which  is  attached  to 
the  sub-stage.  As  will  be 
noticed  in  the  illustration, 
the  lower  surface  of  the 
paraboloid  is  plane,  ^and 
the  Hght  passes  through 
this  without  undergoing  any 
change.  When  it  reaches 
the  polished  parabolic  sur- 
face it  is  reflected  to  one 
point,  according  to  the 
simple  optical  law  that  the 
Fig.  19.  angle   of  reflection  is  equal 

to  the  angle  of  incidence.  An  opaque  stop,  which 
is  cemented  to  the  concave  surface,  prevents  the 
hght  from  passing  through  the  central  portion  of  the 
paraboloid.  The  object  is  thus  illuminated  on  all 
sides  by  such  an  obliquity  of  light  that  it  does  not 
pass  into  the  objective;  the  object  stands  out  in 
relief,  pleasantly  illuminated  on  a  dark  back-ground. 
In  using  the  paraboloid,  the  plane  mirror  should  be 
used,  and  it  is  necessary  to  vary  its  distance  from  the 
object    in    order    to    attain    the  best   results. 

Polarized  Light.— There  is  still  another  method 
of   illumination.     The    light  from    the    mirror  is  polar- 


J 


/V/ 

\/C\ 

/  '*''    ^^~ 

^\     **I  \ 

/A  /  ^^ 

■>  T'-X  ' 

1 

^  X 

i'\N 

/, 

t            ' 

% 

I     **  I 

\ 

\  \ 

■-\ 

t 

1 

58 


ized  (undergoes  a  change)  by  passing  through  two 
Nicol  prisms,  one  of  which  is  fastened  into  a  re- 
volvable  mounting  in  the  sub-stage,  and  is  called  a 
polaryzer.   (Fig.  20.)  The  other  is  in  a  mountmg  above 


Fig.    20.  Fig.  21. 

the  objective  is  called  an  analyzer; — (Fig.  21) — both 
in  conjunction  are  called  a  polai'iscope.  Although 
this  is  not  an  accessory  for  general  purposes,  it  is 
nevertheless  one  which  is  very  popular,  and  produces 
the  most  striking  display  of  brilliant  colors  imagin- 
able. Although  it  is  strictly  a  scientific  apparatus 
for  use  on  crystals,  it  may  be  made  the  means  of 
passing  extremely  pleasant  and  profitable  hours.  Any 
object  in  a  crystalline  form  is  suited,  also  sections  of 
rock.  Another  interesting  study  with  this  apparatus  is 
the  observance  of  the  crystallization  of  table-salt,  as  the 
water  evaporates,  after  it  was  first  dissolved. 

Cover-Glass.— Thus  far  no  attention  has  been 
given  to  the  use  of  the  cover-glass,  although  it  is  an 
important   factor   in    reaching    good    results.     In    pre- 


59 

liminary  examinations  of  solid  objects  with  low 
powers  it  may  be  dispensed  with ;  but  where  fluids 
are.  used,  whether  with  low,  medium,  or  high  powers, 
it  should  always  be  used.  A  drop  or  small  quantity 
of  fluid  placed  upon  a  slide  assumes  a  spherical 
form,  and,  on  viewing  it  with  a  low  power,  it  will 
be  found  to  give  a  distorted  field,  and  will  cause 
disagreeable    reflections   and    shadows. 

As  stated  before,  medium  and  high  powers  have  a 
comparatively  short  working  distance,  and  the  front 
lenses  will  be  so  close  to  the  water,  urine,  blood, 
etc.,  that  the  capillary  attraction  will  often  cause 
an  adherence  to  the  front  surface  of  the 
objective  ;  besides  this,  there  is  such  a  considerable 
depth  to  the  fluid  that  it  obstructs  the  light,  re- 
quires a  great  change  in  adjustment  for  the  various 
planes,  and  is  usually  in  such  vibration  that  a  sharp 
focus  becomes  impossible ;  by  merely  dropping  a 
cover-glass    upon   it    all   these  objections  are  overcome. 

If,  however,  this  does  not  give  sufficient  space 
between  the  sHde  and  cover,  as,  for  instance,  in  the 
observation  of  liv- 
ing forms,  a  live 
cage — (Fig.  22)— 
o  r  coi7ipressor  — 
(Fig.    23) — should  Fig.   22. 

be  used.  In  either  of  these  the  space  may  be  in- 
creased at  will ;  in  the  first  by  a  sliding  arrange- 
ment,  in    the   second   by  means    of  a   screw. 


60 


Fig.   23. 

The  above  are  merely  practical  considerations,  but 
there  are  others  of  a  theoretical  nature  and  of  as 
much  importance.  After  a  high  power  objective  has 
been  corrected  to  a  certain  thickness  of  cover,  anv 
variation,  not  necessarily  considerable,  has  an  injuri- 
ous effect  upon  the  spherical  corrections,  and  conse- 
quently upon  the  resolving  power.  It  is  manifest 
that  the  quaHty  of  the  latter  will  decrease  as  the 
variation  increases,  and  when  it  reaches  a  point  where 
no  cover  is  used,  it  may  be  so  considerable  as  to 
destroy  an  accurate  perception  of  what  is  sought. 


In  this  connection  it  ii^,  considered  important  to 
state  what  thickness  of  cover-glass  it  is  best  to  use. 
As  is  probably  well  known,  there  are  three  grades, 
which  are  designated  as  No.  t,  No.  2  and  No.  3. 
Although  they  are  classified,  those  of  the  same 
number  are  not  absolutely  of  one  thickness.  The 
variation  is  about  as  follows :  No.  i,  1-150  —  i- 
200  inch  thick;  No.  2,  i-ioo —  1-150  inch  thick; 
No.    3,     1-50  —  i-ioo    inch    thick.     According    to    the 


61 

prices  of  cover  -  glasses,  when  purchased  by  weight, 
the  No.  I  give  the  greatest  number  and  No.  3 
the  least.  It  may  for  this  reason  be  thought,  that 
the  purchase  of  No.  i  is  most  advantageous,  but 
it  must  be  considered  that  there  is  a  greater  pro- 
portion of  breakage  by  cleaning,  as  they  are  very 
thin  and  sensitive.  Considered  only  from  an  opti 
cal  standpoint.  No  2  should  generally  be  used,  as 
the  medium  and  high  power  objections  are  adjust- 
ed to  this  thickness  and  give  the  best  results  with 
the  thinnest  of  these.  The  same  thickness  is  also 
used  on  test  objects,  but  they  are  generally  not 
of  as  much  uniformity,  as  might  be  desiied.  Ob- 
jectives sometimes  have  such  an  extremely  short 
working- distance,  that  it  is  necessary  to  use  the 
thinnest  of  No.  i,  but  as  these  are  usually  pro- 
vided with  adjustment  for  correction,  their  injurious 
influence  is  not  so  much  felt.  The  thickest  covers 
are  most  comfortable  to  handle  and  may  be  used 
with  low  power  objectives  without  much  sacrifice 
of   definition. 

* 
To    Draw    Objects.— It     is    very    important    that 

the  appearance  of  an  object  should  be  put  upon 
paper,  especially  of  one  which  is  not  permanently 
mounted.  To  do  this  does  not  require  any  great 
amount  of  skill,  as  the  lines  which  are  projected 
upon  paper  are  merely  followed  out ;  but  it  is  neces- 
sary that  those  drawings  be  made  trutliful.  Nothing 
should    be    put    down     which     is    not    actually   seen ; 


6^ 


neither  should  anything  be  omitted.  Drawings  thus 
made  form  a  valuable  record,  not  only  for  the  in- 
dividual, but  for  others  who  are  following  the  same 
line  of  study. 


Camera  Lucicla— How  to  Use  It. — The  camera 
lucida  is  the  apparatus  by  means  of  which  drawings 
are  made.  There  are  various  forms  of  these,  but 
those  in  most  general  use  are  the  Wollaston  Prism 
and  neutral  tint  glass.  The  methods  of  attaching  to 
the  tubes  are  also  numerous,  but  a  very  simple  and 
effective     device    is     that     shown     in    Fig.     24.     The 


Fig.   24. 


63 


mounting  is  made  of  hard  rubber,  and  is  fixed  to 
the  cap  of  the  eye-piece  by  means  of  a  flexible, 
grooved    ring. 

The  procedure  of  working  should  be  about  as 
follows :  Focus  upon  the  object  and  then  incline  the 
body,  so  that  the  center  of  the  eye-lens  will  be  lo 
inches  from  the  table.  To  obviate  repeated  measure- 
ments, a  standard  stick  of  this  length  may  be  used. 
If  the  instrument  is  so  low  that  it  will  not  allow 
the  inclination  of  the  body  to  an  angle  of 
at  least  45  degrees  when  at  this  distance, 
it  should  be  placed  upon  a  box;  or,  if  not  too 
high,  upon  the  case  of  the  microscope.  Now  read- 
just the  mirror  and  attach  the  camera  lucida  from 
below  and  place  the  paper  under  the  instrument ; 
look  into  the  camera  lucida  from  above,  being  care- 
ful that  the  eye  is  directly  over  the  center  of  its 
opening,  and  the  image  will  be  found  to  be  pro- 
jected upon  the  paper.  Possibly,  and  very  probably, 
it  will  appear  faint.  This  is  due  to  the  fact  that 
the  paper  is  almost  as  highly  illuminated  as  the  field. 
To  remedy  this  defect  a  cardboard  should  be  placed 
between  the  paper  and  light,  so  that  the  former  will 
be  shaded  ;  the  object  will  now  come  out  in  strong 
contrast.  Take  a  well  pointed  pencil  and  follow  the 
lines  in  the  image.  A  little  difficulty  may  at  first 
be  experienced  in  seeing  both  the  pencil  and  image 
at  one  time,  but  after  a  little  practice  this  is  over- 
come. 


64 

It  very  often  occurs  that  the  pencil  point  can  on 
no  condition  be  seen  distinctly,  but  this  is  usually 
due  to  abnormal  sight,  with  which  persons  are 
often  afflicted.  In  these  .  cases,  the  glasses  which 
are  required  for  reading  should  also  be  used  in 
drawing.  The  difficulty  is  not  experienced  in  the 
image,    as    this   can    be   adjusted    to  the  eye. 

DeteriniiiiDg  the  Mngnitying:  Power.— Although 
a  magnifying  table  may  be  furnished,  this  gives  the 
powers  merely  approximately,  as  more  or  less  varia- 
tion occurs  in  objectives  and  eye  pieces  of  the  same 
kind.  As  it  is  interesting,  and  sometimes  important, 
to  know  the  exact  magnifying  power,  a  simple  method 
is  mentioned.  Procure  a  stage  micrometer^  divided 
into  i-ioo  inch  and  i-i,ooo  inch,  and  perhaps 
1-5,000  inch,  or,  if  preferred,  any  suitable  division 
in  milhmetre.  Place  the  micrometer  on  the  stage, 
focus  and  incline  the  microscope,  as  if  for  drawing, 
to  within  10  inches  from  the  table  and  attach  the 
camera  lucida ;  for  low  powers,  i-ioo  divisions  may 
be  used:  for  the  higher  ones,  i-i,ooo  or  more;  the 
divisions  as  now  projected  may  be  marked  upon  the 
paper  and  then  measured  off  with  a  rule  divided 
into  inches  and  i-io  inches;  if,  for  instance,  the 
1-1,000  divisions  are  used,  and  one  division  on  paper 
covered  i  inch  on  the  rule,  it  is  evident  that  the 
magnifying  power  is  1,000  times;  if  it  covered  2-10 
(=1-5)  on  the  rule,  it  would  be  200  diameters, 
and  so  on. 


65 


Measuring  the  Size  of  an  Object —A  simple 
and  reliable  way  of  learning  the  size  of  an 
object  is  by  means  of  an  eye-piece  micrometer.  As, 
however,  this  does  not  measure  the  object  directly, 
but  only  its  image,  the  first  part  of  the  process  makes 
it  more  complicated.  However,  this  portion  is  usually 
attended  to  by  the  manufacturers.  The  eye-piece  is 
provided  with  a  slot,  into  which  a  micrometer  is 
fitted.  A  micrometer  with  the  same  divisions  as 
the  eye-piece  micrometer  is  placed  upon  the 
stage  and  the  objective  focussed  upon  it.  It  is  now 
observed  how  many  of  the  divisions  of  the  eye- 
piece micrometer  are  contained  in  the  magnified 
division  of  the  stage  micrometer,  and  the  resulting 
figure  is  placed  in  the  sub-division  under  the  objec- 
tive. To  determine  the  actual  size  of  an  object,  this 
is  now  placed  on  the  stage  and,  noting  the  number 
of  divisions  which  cover  it,  these  are  divided  by  the 
number  on  the  card,  and  the  resulting  figure  gives 
the  actual  size.  Suppose  the  figure  on  the  card  is  8.0 
and  the  image  of  the  object  covers  40  of  the  spaces 
which  are  divided  into  i-io  milli-metre,  the  size  of 
the  object  would  be  5-10,  or  1-2  milli-metre;  or, 
expressed  in  inches,  (25.4  m.  m.  =  i  inch)  about  1-50, 


ADVANCED    MANIPULATION. 


Dry  Adjustable  Objectives.— The  information 
which  has  thus  far  been  given  on  the  manipulation 
of  the  microscope  may  be  termed  initiatory,  as  it  is 
supposed  (at  any  rate  hoped)  to  have  disclosed  some 
new  principles.  These  are  comparatively  simple,  and^ 
with  a  moderate  amount  of  attention,  are  easily 
acquired.  It  is  the  intention  now  to  speak  of  some- 
thing more  complex  and  to  give  instruction  in  the 
use  of  higher  grade  adjustable  and  immersion  objec- 
tives. The  difficulty  of  doing  this  increases  with 
each  step  of  advance,  and  whether  it  can  be  over- 
come by  means  of  written  words,  is,  perhaps,  an 
open  question.  However,  the  writer  is  certain  that 
if  the  following  instructions  are  faithfully  adhered  to, 
satisfactory  results  will  be  gained.  The  highest  attain- 
ment must  of  necessity  be  the  result  of  perseverance 
and  knowledge  of  the  various  properties  of  an  object- 
ive, which  are  given    in    preceding    pages. 

It  is  assumed  that  a  dry  objective  is  used,  say  a 
1-6  140°  or  1-8  135°,  and  provided  with  screw- 
collar  adjustment,  for  cover-glass  thickness ;  it  is  fur- 
ther assumed  that  in  the  first,  the  variation  between 
the  two  first  systems   (anterior  and  middle)  is  attained 


G7 


l)y  means  of  a  rectilinear  motion  to  the  middle  and 
l)osterior  system  and  stationary  anterior  system,  while 
in  the  second  tl\e  conditions  are  reversed ;  the  two 
posterior  systems  remain  stationary  while  the  front  is 
adjusted.  Both  are  arranged  with  graduations  upon 
the  screw-collar  and  have  an  index.  In  both  zero 
represents  the  adjustments  as  open^  i.  e.,  the  lenses 
are  separated  to  their  fullest  extent,  and  the  objec- 
tive is  approximately  adjusted  for  the  thinnest  covers. 
As  the  adjustment  is  moved  towards  the  higher 
numbers,  it  is  closed^  and  indicates  correction  for 
gradually  thicker  covers.  At  the  medium  point  5, 
the  correction  is  about  for  medium  thickness,  while 
at  9  it  is  for  the  thickest.  Before  the  objective  is 
attached  the  adjustment  should  be  closed,  as,  if  this  is 
neglected  and  the  objective  has  a  short  working  dis- 
tance, the  front  lens  may  come  in  contact  with  the 
cover  when  it  is  focused. 

Probably  the  best  object  for  studying  the  effect  of 
the  screw-collar  adjustment  and  acquiring  skill  in  de- 
termining its  best  point  is  again  P.  angtilaiiwi. 
Place  a  slide  of  this  upon  the  stage,  and  with  a 
low  power  eye-piece  select  a  diatom  which  appears 
to  be  flat  j  such  a  one  may  usually  be  found  when 
there  are  a  number  on  the  slide.  After  the  ob- 
jective has  been  attached  to  the  nose-piece,  focus  care- 
fully and  observe  whether  any  lines  can  be  seen ;  if  not, 
grasp  the  milled  edge  of  the  adjuslmeni  collar  between 
the  thumb  and  first  finger  of  the  left  hand,  keeping  the 
fingers   of  the  right  hand   upon  the  micrometer  screw, 


68 


Of  vice  versa,  if  from  the  outset  it  was  made  a  habit 
to  use  the  left  hand  on  the  fine  adjustment;  turn 
the  collar  slightly  toward  its  open  point,  and  as  this 
will  place  the  object  out  of  focus,  move  the  fine 
adjustment  correspondingly;  continue  to  turn  the  col- 
lar, little  by  little,  and  do  not  cease  to  observe 
closely ;  also,  after  each  movement,  focus  above  or 
below  the  plane  of  the  object,  so  that  this  will  be 
indistinct,  and  look  for  the  lines.  Possibly  after  a 
little  they  will  begin  to  appear  faintly;  but,  if  not, 
continue  to  bring  the  collar  toward  the  middle.  The 
lines  must  now  soon  make  their  appearance,  and, 
when  they  do,  it  will  probably  be  above  the  plane 
of  the  diatom.  This  is  an  indication  that  the  objec- 
tive is  approaching  its  correction  for  the  cover.  Now 
keep  the  /i?tes  in  focus,  while  the  correction  collar 
is  being  gradually  turned,  until  the  fines  and  the 
outline  of  the  diatom  lie  in  one  plane ;  the  objec- 
tive is  now  said  to  be  co7''rected  for  cover.  Observe 
which  number  corresponds  to  the  index,  and  note 
this  upon  paper;  again  return  the  collar  to  its 
closed  point  and  go  through  the  same  proceeding  as 
carefully  as  at  first.  When  the  best  point  is  again 
reached,  look  for  the  number  and  see  whether  it 
agrees  with  the  first ;  very  likely  it  does  not,  which 
is  owing  to  a  want  in  the  faculty  of  perception,  due 
to  a  too  slight  acquaintance  with  the  phenomena. 
These  trials  should  be  repeated  until  the  proper  sen- 
sitiveness of  feeling  in  making  the  adjustments  is 
acquired,  and  until  they  can  be  made  to  correspond 
with   a  certainty   to   within  at  least  two  divisions. 


69 


Remove  the  eye-piece  and  attach  one  of  higher 
power.  It  must  now,  however,  be  remembered  that 
if  there  is  a  considerable  difference  in  the  powers, 
there  will  be  a  relative  difference  in  their  lengths, 
and  that  this  will  cause  a  difference  in  the  optical 
length  of  the  tube  ;  this  not  only  will  require  another 
adjustment  for  focus,  but  will  partially  destroy  the  cor- 
rection as  made  with  the  low  power.  After  some 
practice,  the  amount  of  variation  may  be  fixed  upon 
and  may  be  noted  for  the  future  ;  but,  to  determine 
it,  the  same  plan  as  suggested  with  the  low  power 
eye-piece  should  be  followed. 

When  it  is  found  after  repeated  trials  that  suffi- 
cient skill  has  been  acquired  to  bring  the  collar  to 
within  one  division,  the  number  and  power  of  the 
eye-piece  should  be  scratched  with  a  diamond  upon 
the  slide  or  with  pen  and  ink  upon  the  label ; 
thus,  if  it  is  found  that  with  a  i^^  inch  eye-piece  the 
index  shows  5,  and  with  a  J  inch  eye-piece  shows 
5 J,  it  should  be  marked  i^ — 5  and  J — 5^,  For 
future  examinations  on  the  same  slide,  this  will  facili- 
tate work  and  give  the  assurance  that  the  best  results 
are    thus    gained    without  further  trial, 

Mr.  Wenham's  general  rule  for  obtaining  the  best 
correction  on  objects  in  general  is  as  follows  : 
"  Select  any  dark  speck  or  opaque  portion  of 
the  object  and  bring  the  outline  in  perfect  focus ; 
then  lay  the  finger  on  the  milled  head  of 
the    fine      adjustment     and      move    it    briskly    back- 


70 


wards  and  forwards  in  both  directions  from  the 
first  position.  Observe  the  expansion  of  the  dark 
OLitHne  of  the  object,  both  when  wi'.hin  and  when 
without  the  focus.  If  the  greater  expansion  or  coma 
is  when  the  object  is  without  the  focus  or  farthest 
from  the  objective,  the  lenses  must  be  placed  farther 
assunder  (or  opened).  If  the  greater  coma  is  when 
the  object  is  within  the  focus,  or  nearest  the  objec- 
tive, the  lenses  must  be  brought  closer  together  (  or 
closed  ).  When  the  objective  is  in  proper  adjustment, 
the  expansion  of  the  outline  is  the-  same  both  within 
and  without  the  focus." 

liiiniersioii-Adjustable      Objectives. —  As     was 

stated  before,  immersion  contact  between  the  objective 
and  cover  glass  is  made  by  either  water  or  homo- 
geneous fluid.  The  fluid  should  be  kept  in  a  small 
bottle  or  phial,  the  cork  of  .which  is  pierced  to  receive 
a  small  pointed  stick  or  match,  and  this  should  pro- 
ject sufficiently  so  that  it  will  enter  the  fluid  about 
J-inch.  The  fluid  will  then  always  be  free  from  dust, 
and  by  withdrawing  the  cork  the  stick  will  always 
carry  a    drop    of  fluid    with    it. 

In  fixing  an  immersion  objective  to  the  stand, 
the  latter  should  first  be  put  in  an  upright  position; 
the  fluid  should  now  be  attached  to  the  front  lens, 
but  care  should  be  taken  not  to  put  on  too  much; 
it  should  be  merely  enough  to  cover  the  surface. 
If  too   much,    portion    of    it    should    be   removed   by 


71 


allowing  it  to  adhere  to  the  finger.  The  objective 
may  then  be  attached  to  the  stand  and  brought  down 
until  the  fluid  is  in  contact  with  the  cover ;  the 
stand  is  now  inclined  and  the  objective  focused;  if 
this  method  is  followed  there  is  no  danger  of  flooding 
the  entire  cover  with  fluid,  which  sometimes  may  be 
the  means  of  destroying  the  object;  neither  can  the 
fluid  run  out  from  between    the    two    surfaces. 

Extreme  cleanliness  should  be  observed  in  all 
work  connected  with  the  microscope,  and  particularly 
in  the  use  of  immersion  objectives.  The  use  of  im- 
mersion fluid  in  itself  involves  a  certain  amount  of 
inconvenience,  but  as  in  many  cases  it  is  absolutely 
necessary,  the  observance  of  fixed  rules  will  materi- 
ally help  to  overcome  some  of  the  disagreeable  fea- 
tures. After  the  work  with  an  immersion  objective  has 
been  completed,  the  objective  should  be  removed 
from  the  stand  and  its  front,  as  well  as  the  slide, 
should  invariably  be  cleajied ;  the  fluid  may  be  re- 
moved  by  a  moist  piece  of  soft  linen  and  then  cleaned 
with  a  dry  piece  ;  chamois  skin  is  not  suitable,  as  it 
does  not  absorb  the  fluid. 

Test-Plate.—  Almost  all  microscopists  who  take 
an  active  interest  in  the  capacity  of  their  instruments, 
supply  themselves  with  a  set  of  test  objects^  of 
which  P.  aiigulatiwi  is  in  most  general  use  or  with 
a  so-called  test-plate.  These  plates  consist  either  of 
a   series    of  bands    of   finely   ruled    lines  ranging  from 


72 


5,ooo  to  the  inch  to  120,000  to  the  inch  (beauti- 
ful specimens  of  these  are  made  by  Prof  Wm.  H, 
Rogers,  of  Cambridge,  Mass.,  and  C.  Fasoldt  of 
Albany,  N.  Y.,  )  or  with  a  series  of  diatoms,  upon 
which  the  markings  represent  certain  divisions  of  an 
inch.  The  one  of  these  which  is  principally  used 
is  made  by  J.  D.  Moeller  and  consists  of  a  series 
of  20  diatoms.  They  are  furnished  mounted  both  ^/r>' 
and  iu  balsa77i,  but  the  latter  is  the  most  common. 
Below  is  a  table  giving  the  numbers,  names  of  the 
various  diatoms  and  divisions  on  their  surfaces  to 
1-1,000  inch.  A  specimen  of  Eupodiscus  Argus 
begins    and    ends   the    series : 


1.  Triceratium    Favus    Ehrbg 

2.  Pinnularia   nobilis    Ehrbg 

3.  Navicula    Lyra  Ehrbg.    var 

4?  Navicula   Lyra  Ehrbg 

5.  Pinularia    intenupta    Sm.    var.... 

6.  Stauroneis    Phoenicenteron    Ehrbg 

7.  Grammatophora    marina    Sm   , . .  . 

8.  Pleurosigma   balticum    Sm 

9.  Pleurosigma     acuminatum     (  Kg.) 

Grun 

10.   Nitzschia  Amphioxys    Sm 

IT.  Pleurosigma   angulatum    Sm 


Striae 

m 

I -1000 

of  an    inch. 

•  3-1 

to 

4. 

II. 7 

to 

14. 

14-5 

to 

18. 

23- 

to 

30-5 

25-5 

to 

29-5 

31. 

to 

36.5 

36. 

to 

39- 

32. 

to 

37- 

41. 

to 

46.5 

43- 

to 

49. 

44. 

to 

49. 

73 


12.   Grammatophora    oceanica    Ehrbg. 
=G.    subtilissima 


13 


Surirella    Gemma    Ehrbg 

14.  Nitzschia    sigmoidea    Sm 

15.  Pleurosigma    Fasciola    Sm.   var, 

16.  Surirella    Gemma    Ehrbg 

17.  Cymatopleura    elliptica    Breb.. 


60. 

to  67. 

43 

to  54. 

61 

to  64. 

55- 

to  58. 

64 

to  69. 

55 

to  81. 

78. 

to  87. 

83- 

to  90. 

92. 

to  95. 

18.  Navicula  crassinervis  Breb.=Frus- 

tulia   saxonica    Rabh 

19.  Nitzschia   curvula    Sm 

20.  Amphipleura   pelliicida    Kg 


It  may  be  said,  and,  perhaps  with  truth,  that  a 
test  plate  does  not  belong  to  the  necessities  of  an 
outfit,  but  considering  that  it  is  a  guage,  on 
which  the  optician  usually  bases  the  quality  of  his 
objectives,  it  is  valuable  to  the  owner  of  an  objec- 
tive to  be  able  to  determine  whether,  under  his 
manipulation,  the  objective  will  perform  as  well  as  is 
claimed  for  it ;  due  consideration  must,  however, 
be  given  to  the  fact  that  there  is  a  certain  amount 
of  variation  among  different  plates,  as  is  shown  in 
the  above  table..  Outside  of  this,  it  is  a  continual 
incentive  to  determine  the  extreme  performance  of 
an  objective  and  it  thus  becomes  the  means  of  ac- 
quiring great  manipulative  skill,  which  can  not  be 
underrated.  The  writer  is  in  a  position  to  know 
that  there  is  great  need  of  this ;  innumerable  cases 
have    come    to   his    notice     where    several    objectives 


74 

of  the  same  kind  and  equal  quality  gave  unequal 
results  in  different  hands,  and  would  be  highly- 
eulogized  by  the  possessor  of  one  and  condemned 
by   that    of   another. 

Immersion  Objectives  on  Test  Plate.— To  de- 
termine the  highest  capacity  on  test  objects,  ordinary 
day  hght  is  hardly  sufficient ;  moderate  sun  light  or' 
good  lamp  light  is  best  suited,  but  the  latter,  from 
the  fact  that  it  is  always  at  hand,  is  preferable.  For 
the  purpose  of  explanation^  we  will  assume  that  a 
flat-wick  lamp  and  a  i-8  or  i-io  homogeneous  im- 
mersion objective  is  used.  If  the  right  hand  is  used 
on  the  micrometer  screw,  place  the  lamp  at  the  right 
side  of  the  instrument,  about  lo  inches  from  it, 
with    the  edge  of  the    flame  turned  toward  the  mirror. 

The  test  plate  may  now  be  placed  upon  the  'stage, 
and  as  the  diatoms  in  balsam  are  very  transparent, 
and  therefore  very  difficult  to  find,  a  lower  power  ob- 
jective may  be  used  as  a  finder;  bring  No.  i,  or 
Triceratiiim  Favus^  in  the  center  of  the  field,  and 
after  the  objective  has  been  removed,  attach  the 
immersion  objective  in  the  manner  prescribed  ;  the 
adjustment  collar  may  be  placed  at  zero,  as  this  is 
about  the  correct  point  for  standard  length  of  tube. 
Get  the  best  possible  illumination  with  the  mirror  at 
the  central  point  and  move  the  test  plate  from  dia- 
tom to  diatom  until  it  reaches  No.  ii,  P.  angula- 
tion, but  observe  closely  the  structure  of  each  one 
as  it  comes  into  the  field.  Next  see  whether  the 
objective   is    corrected ;    if   the    fines   and   outline,  or 


75 


middle  rib,  do  not  appear  to  be  in  one  plane,  ad- 
just the  collar  until  they  are,  and  then  continue 
the  advance  toward  the  higher  numbers  until  one  is 
reached  on  which  no  lines  can  be  seen.  Swing  the 
mirror-bar  to  an  obliquity  of  20*^.  and,  readjusting 
the  mirror,  observe  the  effect.  It  is  very  probable 
that  the  lines  will  show,  and,  if  so,  continue  the  ad- 
vance ;  if  they  do  not,  give  10°  or  20°  more  obliquity, 
and  after  the  structure  comes  out,  again  go  forward.  A 
point  may  thus  be  reached,  where  with  the  greatest 
obliquity  which  can  be  given  and  with  the  best 
possible  illumination,  the  objective  seems  to  have 
come  to  the  limit  of  its  performance.  From  the 
claims  which  have  been  made  for  it,  it  ought  to  do 
better.  What  is  the  cause  of  failure  ?  Possibly  the 
mirror  is  not  correctly  focused,  or  the  adjustment 
collar  may  not  be  correct  for  oblique  light  ;  perhaps 
the  eye-piece  does  not  give  sufficient  magnifying 
power  to  distinguish  the  stri^.  It  may  be  any 
one  of  these  causes  or  all  combined.  As  to  the  eye- 
piece ,  the  manipulator  must  remember  the  amount 
of  separation  of  lines  in  the  last  object  which  was 
resolved,  and  from  the  gradation  in  the  coarser 
specimens  must  judge  whether  the  power  is  suffi- 
cient ;  it  should  be  added  that  for  any  over  No.  14 
and  under  No.  18  a  J  inch  eye-piece  should  be 
used,  and  for  those  above  No.  18  a  power  of  ^ 
inch  will  probably  be  necessary,  provided  a  J  or 
one-tenth  objective  is  used.  After  this  condition 
has    been    complied    with,   look  to    the   correction  col- 


76 


lar  of  the  objective ;  to  obtain  the  highest  results 
.it  very  often  occurs  that  a  different  adjustment  is 
required  for  obHque  Hght  from  that  for  central  light. 
Note  the  number  at  which  it  stands,  and  then  work 
it  back  and  forth,  watching  carefully  for  results.  If 
this  has  no  influence,  return  it  to  its  number  or  to 
a  point  where  the  outline  of  the  object  appears  most 
sharp.  Now  look  to  the  illumination  ;  vary  the  dis- 
tance of  the  mirror  to  the  object,  and,  if  it  con- 
flicts with  the  stage  or  does  not  give  the  desired 
results,  vary  the  distance  of  the  lamp  to  the  instru- 
ment and  watch  the  efl"ect  of  the  change  through 
the  tube.  A  great  change  in  the  illuminating  power 
can  thus  be  produced ;  the  light  is  best  when  it  cov- 
ers the  least  space,  as  it  is  then  most  intense.  The 
hght  may  be  quickly  adjusted  by  throwing  it  upon 
a  point  on  the  shde  in  the  opening  of  the  stage 
and  watching  it  there.  If  neither  of  these  changes 
give  any  improvement,  recourse  must  be  had  to  an- 
other expedient.  Place  a  bulls-eye  between  the  lamp 
and  the  mirror  in  such  a  position  that  the  light 
will  be  well  thrown  upon  the  latter.  For  this  pur- 
pose it  should  be  moved  back  and  forth.  Keep  it 
a  little  below  the  line  of  the  face  of  the  stasre,  so 
that  the  light  will  not  strike  it  on'  its  upper  and 
as  Httle  as  possible  on  its  lower  surface ;  if  the 
light  from  the  bulls-eye  directly  reaches  the  object, 
it  destroys  the  eftect  of  the  oblique  illumination. 
Great  care  should  be  given  to  this  point,  as  it  is 
very  important. 


77 

if  all  of  these  suggestions  have  been  followed,  a 
great  difference  will  undoubtedly  be  noticed  in  the 
performance  of  -the  objective  ;  but  if  it  still  does  not 
come  up  to  the  standard,  patience  must  not  be  lost. 
The  slightest  change  in  the  mirror,  bulls-eye,  or 
lamp,  a  touch  to  the  correction  collar  or  micro- 
meter screw  is  sometimes  followed  by  astonishing 
results.  The  beginner  should  sit  down  with  the  ex- 
pectation that  he  will  fail  at  the  first  trial.  At 
each  succeeding  trial  he  can  easily  notice  his  im- 
provement in  manipulation  and  the  gain  of  corre- 
sponding results.  He  should  be  able  to  bring  the 
performance  of  the  objective  up  to  the  claims  made 
for  it,  if  it  has  come  from  the  hands  of  a  reliable 
optician,  and  should  not  rest  until  this  is  accom- 
pHshed. 

To  the  histologist  it  may  seem  strange  that  the 
writer  has  thus  far  only  spoken  of  working  with 
objectives  on  diatoms.  This,  however,  was  done 
advisedly.  They  are  thin,  and  therefore  as  suitable 
as  a  thin  section  and  far  more  preferable  than  a 
thick  one.  Their  form  and  structure  is  easily  recog- 
nisable, and  there  is  very  little  variation  among 
those  of  the  same  kind ;  therefore,  rules  laid  down 
regarding  them  are  generally  good.  It  is  conceded 
by  advanced  workers  that  the  time  spent  over  dia 
toms  for  the  purpose  of  studying  objectives  is  well 
applied,  and  the  most  expert  manipulators  have  ac- 
quired   their   experience    in    this    manner.     An    objec- 


78 


tive  which  works  well  on  diatoms  works  equally  well 
on  other  objects,  and  therefore  the  manipulative 
skill  which  has  been  attained  on  the  former  is  as 
well  apphed  on  the  latter.  At  the  outset,  work  may 
be  done  on  other  objects  than  diatoms,  and  where 
ordinary  working  objectives,  such  as  a  student  i  inch 
and  1-4  inch,  or  3-4  and  1-5  inch  comprise  the 
outfit,  the  road  to  good  manipulation  may  be  as 
short  as  with  diatoms.  The  conditions  in  both  cases 
remain  the  same ;  but  it  must  be  cautioned  that, 
if  histological  preparations  be  used,  only  such  be 
selected  as  are  reliable.  A  poor  specimen  is  per- 
haps as  bad  as  none  at  all ;  an  abnormally  thick  one 
obstructs  hght,  makes  it  impossible  for  the  objective 
to  penetrate  through  the  various  layers,  and  leaves 
the  impression  that  the  latter  is  defective. 


SUB-STAGE    ILLUMINATION. 


This  chapter  should  form  an  important  part  in  a 
book  of  instruction  on  the  use  of  microscope.  For 
many  years  after  the  discovery  of  the  achromatic 
objective,  sub  stage  iHuminators  formed  an  important 
part  of  an  instrument,  as  the  loss  of  light  with 
medium  and  high  powers  was  considerably  greater 
than  it  is  to-day;  also  because  the  method  of 
mounting  the  mirror  was  more  primitive  than  it  is 
at  present.  Then  after  new  methods  were  devised 
for  increasing  the  angular  aperture  and  thus  attain- 
ing a  higher  performance  in  objectives,  the  neces- 
sity for  it  was  not  so  much  felt,  and  it  came  into 
disuse  to  a  certain  extent.  Of  late  years,  however, 
the  demand  for  it  has  again  sprung  up,  but  in  a 
form  to    meet    present   requirements. 

By  means  of  a  condenser  of  proper  construction, 
better  results  are  reached  than  by  the  mirror 
only,  and  although  a  certain  amount  of  delicacy  of 
manipulation  is  necessary,  the  best  performance  of 
the  objective  is  accomplished  more  easily  with  one 
than  without  one.  Very  often  results  may  be  gain- 
ed with  one  of  good  construction,  which  it  is  im- 
possible   to   reach   without    one. 


80 

ObjectiA-es  and  Eye  Pieces.— A  medium  power 
objective  does^^good  service  as  a  sub-stage  condenser, 
but  its  length  precludes  its  general  use.  To  attach 
it  to  the  microscope  it  should  be  screwed  into  a 
sub-stage  adapter  with  society  screw  and  fitted  to 
the  sub-stage  with  its  front  system  toward  the 
object. 

The  Periscopic  eye-piece  is  similar  in  its  con- 
struction to  the  so-called  Webster  condenser,  and 
gives  excellent  results.  It  is  attached  in  the  same 
manner  as  an  objective,  except  that  a  special 
adapter  must  be  provided.  In  the  Professional  micro- 
scrope  of  the  Bausch  &  Lomb  Opt.  Co.,  the  diam- 
eter of  tube  corresponds  to  that  of  the  sub-stage, 
so  that  the  eye-pieces  can  be  attached  without 
special    fitting. 

Hemispherical ',  Lens. — This  is  probably  the  most 
simple  form,  of  sub-stage  illuminator.  It  is  a  plano- 
convex lens  of  about  ^-inch  diameter,  and  is  at- 
tached directly  to  the  slide  by  means  of  an  immer- 
sion fiuid.  Where  no  great  obHquity  is  employed,  a 
better  plan  is  to  cement  it  to  a  thin  sHde  and 
then  place  the  sHde  with  object  upon  this.  It  is 
not  as  its  shape  might  imply,  a  condenser,  except 
when  the  plane  mirror  is  used,  when  it  partially 
acts  as  such ;  theoretically  the  light  from  the  con- 
cave mirror  should  pass  through  it  without  under- 
going any  change.  A  greater  obHquity  of  Hght  is 
obtained   by   it    than  is  possible    without   it. 


81 


Weiiliaiii  Button.— This  is  the  invention  of  Mr. 
Wenham,  and  is  attached  to  the  sUde  in  the  same 
manner  as  the  hemispherical  lens.  It  is  a  semi- 
circular glass  with  two  flat  surfaces;  the  third  and 
upper  surfaces  are  polished  and  the  circular  one  is 
ground  and  polished  on  a  curve  of  a  shorter 
radius  than  the  circle,  thus  permitting  a  certain 
amount   of   concentration   of  light.    . 

Woodward  Prism.— This  device  was  suggested 
by  the  late  Col.  J.  J.  Woodward,  and  is  attached 
as  mentioned  above.  It  is  a  triangular  prism  and  is 
intended  to  allow  the  use  of  extreme  obHque  light. 
It  is  in  no  sense  a  condenser,  but  merely  permits 
oblique  light  to  reach  the  object  without  undergo- 
ing  refraction. 

Narrow     Angle     Condensers. —These     may     be 

divided  into  two  classes — achromatic  and  non-achro- 
matic. In  effect  they  are  similar  to  objectives,  but 
are  better  adapted  to  the  purpose,  from  the  fact 
that  they  are  provided  either  with  revolving  dia- 
phragms or  with  caps  to  decrease  the  aperture  and 
thus  the  amount  of  Hght.  The  most  simple  forms 
are  of  such  a  length  that  they  can  be  used  on 
instruments,  in  which  the  sub-stage  is  fixed  to  the 
stage,  but  the  more  complex  and  all  those^of  which 
descriptions  follow,  should  be  used  on  adjustable 
sub-stages. 


82 


Wide  Angle  Condensers.— The  most  simple 
form    of   these  is  the  so-called  Im7?iersion   illuminator. 

Although  it  is  intended  to  be  used 
in  connection  with  the  slide  by  fluid 
it  may  also  be  used  dry,  as  may  also 
all  immersion  condensers  ;  but  it 
must  be  remembered  that  it  as 
well    as    others  give  a   greater  amount 

of  light  when  used 
with  immersion  fluid, 
as  all  rays  reach  the 
object,  which  is  not 
^'^-  ^5-  the  case    when    used 

dry.  It  may  be  used  in  plain  adapter,  when  all  the 
light  which  reaches  the  lower  lens  (which  is  nearly 
the  diameter  of  the  Society  screw)  is  concentrated  to 
one  point.  Another  mounting  is  provided,  by  means 
of  which  any  amount  of  -oblique  light  may  be  ob- 
tained, without  changing  the  position  of  the  mirror. 
This  feature  is  particularly  valuable  in  connection 
with  those  stands  in  which  an  extremely  thick  stage 
or  the  construction  of  the  mirror  preclude  any  con- 
siderable   obliquity   of   light. 

Ward's  General  llluniinator.— This  primarily 
is  similar  to  the  above.  The  same  optical  portion 
is  used,  but  it  is  mounted  in  a  sub -stage  fitting.  To 
the  lower  side  of  this  an  Iris  diaphragm  is  fitted  and 
made  to  traverse  by  means  of  a  screw,  from  the  ex- 
treme edge  to  the  center.     The  amount  of  illumination 


83 


©  e 


Fig.  26. 

and  obliquity  is  thus  under  constant  control.  The 
Iris  diaphragm  may  be  removed  and  in  its  place 
may   be   attached    other   adapters    with    various  stops. 

Large  Sub-Stage  CondeDser.—  The  construction 
of  the  optical  portion  of  this  accessory  is  similar  to 
that  of  the  Abbe  condenser,  with  which  Dr.  Koch 
made  his  original  investigations,  and  in  its  results  is  iden- 
tical with  it.  It  is  the  most  approved  form  at  the  present 
time,  not  only  on  account  of  its  wide  angular  aper- 
ture, but  on  account  of  the  extremely  large  size  of  the 
lenses,  which  receive  all  the  light  which  may  pass 
through  the  sub-stage.  It  may  be  used  dry,  but  is 
intended  to  •  be  in  homogeneous  immersion  contact 
with  the  lower  surfaces  of  the  slide.  Three  styles 
of  mounting  are  provided,  one  with  plain  adapter, 
another  with  a  mounting — (No.  i.  Fig.  27), — which, 
by  revolving  a  milled  ring,  causes  a  diametrical  move- 
ment   of  a   J   inch    aperture    from    the    extreme    edge 


84 


Me/Aao.rn/ii». 


No.    I.  Fig.   27.  No.   2. 

to  the  center,  thus  giving  all  grades  of  obliquity  from 
zero  to  the  highest  point.  The  third — (No.  2,  Fig. 
27) — is  provided  with  a  swinging  ring,  which  in  its 
recess  receives  diaphragms  of  various  size  apertures. 
Some  of  these  decrease  the  volume  of  light ;  others 
with  central  stops  give  dark  ground  illumination ; 
another  has  an  eccentric  aperture,  by  means  of  which 
various  grades  of  obliquity  of  light  may  be  attaui- 
ed.  Besides  these  a  blue  glass  is  provided  for 
monochromatic  light  with  lamp  light,  which  may  be 
used  alone  or  in  combination  with  any  of  the  dia- 
phragms. 

This  condenser  is  one  with  which  the  highest  re- 
sults are  gained,  and  which  combines  in  itself  the 
means  of  accomplishing  almost  all  the  various  illumi- 
nating effects. 

In  using  any  of  the  condensers  which  are  attached 
to  the  sub-stage,  different  results  are  gained  by  vary- 
ing their  distance  to  the  object.  This  becomes  neces- 
sary on  account  of  the  difference  in  thickness  in  slides, 
to  properly  focus  the  condenser ;  but  in  many  cases 
the  best  results  are  reached  when  the  light  is  not 
intense.  A  proper  judgment  of  the  best  point  can 
only  be  reached  after  repeated  trials. 


CARE    OF    A    MICROSCOPE. 


Besides  acquiring  tlie  ability  to  ])roperly  use  an 
instrument  with  its  accessories,  it  is  important  to 
know  how  to  keep  it  in  the  best  working  condition. 
It  may  be  said  without  reserve  that'  an  instrument 
properly  made  at  the  outset  and  judiciously  used 
should  hardly  show  any  signs  of  wear  either  in  ap 
pearance  or  in  its  working  parts,  even  after  the  most 
protracted  use;  and  further  than  this,  every  good  in- 
strument should  have  a  provision  for  taking  up  lost 
motion,  if  there  is  a  likelihood  that  this  may  occur 
in    any  of   the  j^arts. 

Especial  care  should  be  given  to  the  optical  parts, 
in  fact  such  care,  that  they  will  remain  in  as  good 
condition  as  when  first  received.  Accidental  injury 
may  of  course  occur  to  them,  but  if  a  systematic 
manner  of  working  is  followed  and  a  special  recept- 
acle for  each  part  is  provided,  this  may  usually  be 
avoided.  The  following  rules  refer  mainly  to  the  in- 
struments manufactured  by  the  Bausch  &  Lomb  Opt. 
Co.,  but  are    applicable    to  instruments    in   general. 

To  Take  Care  of  a  Stand.— One  of  the  first 
rules    should    be    to    keep    the    instrument   free  from 


86 


dust.  This  may  be  done  in  a  manner  formerly 
prescribed.  If  dust  settles  on  any  part  of  the 
instrument,  remove  it  first  with  a  camel's  hair 
brush,  and  then  wipe  carefully  with  a  chamois 
skin,  with  the  grain  of  the  finish  and  not  across  it, 
as  in  the  latter  case  it  is  likely  to  cause  scratches. 
Keep  the  working  and  sliding  parts  absolutely  free 
from  dust,  as  this  grinds  and  will  thus  soon  cause 
play. 

Use  710  alcohol  on  any  part  of  the  instrument,  as 
it  will  remove  the  lacquer.  As  the  latter  is  for  the 
purpose  of  preventing  oxydation  of  the  metals,  it  is 
important  to  observe  this  rule. 

In  using  draw-tube,  impart  a  straight  up  and  down 
motion  to  it.  If  a  spiral  motion  is  given  to  it,  it 
will  cause  scratches,  and  in  time  will  wear  off  the 
nickel. 

If  it  becomes  necessary  to  lubricate  any  of  the 
parts,  use  a  slight  quantity  of  soft  tallow  or  good 
clock  oil. 

In  an  instrument  which  is  in  constant  use,  it  some- 
times occurs  that  the  pinion  works  loose  and  occa- 
sionally to  such  an  extent  that  the  body  drops  of 
its  own  weight.  Tightening  screws  are  provided  to 
take  up  the  play — in  the  Professional,  American  Con- 
centric, Universal,  Griffith  Club  and  Physician  Micro- 
scopes these  are  at  the  back  of  the  pinion.  In  the 
Investigator,  Model  and  Family  Microscopes,  they 
are    seen  in  the  slide   by  removing  the  body. 


87 


In  using  a  screw-driver,  grind  its  two  large  sur- 
faces so  that  they  are  parallel  and  not  wedge-shape, 
and  so  it  will  exactly  fit  in  the  slot  of  the  screw- 
head. 

In  inclining  the  stand  always  grasp  it  at  the  arm 
and  7iever  at  the  tube,  as  in  the  latter  case  it  may 
loosen  the   sHde  or  tear  off  some   of  the   parts. 

When  repairs  or  alterations  are  necessary,  always 
have  these  made  by  the  manufacturers ;  they  can, 
from  the  system  of  duplicated  parts,  not  only  do  it 
cheapest,   but  best. 

To  Take  Care  of  Objectives  and  Kye-Pieces.— 

It  is  as  necessary  to  keep  these  free  from  dust  as 
the  stand,  in  fact  even  greater  cleanliness  should  be 
observed.  When  indistinct,  dark  specks  show  in  the 
field,  the  cause  may  usually  be  looked  for  in  the 
field-lens,  although  sometimes  in  the  eye  lens  also. 
The  dust  may  be  removed  by  a  camel's-hair  brush, 
but  when  this  is  not  sufficient,  use  a  well  washed 
piece  of  linen,  such  as  an  old  handkerchief.  From 
its  fine  texture  chamois  skin  is  desirable,  but  as  it 
is  fatty  it  should  never  be  used  until  after  it  has  been 
well  washed. 

The  same  method  applies  to  cleaning  objectives. 
Clean  an  immersion  objective  invariably  after  it  has 
been  used,  first  by  removing  the  fluid  by  a  moist 
linen  and  then  by  using  a  dry  piece. 


88 


Keep  the  objectives  especially  in  such  a  place 
where  they  are  not  subject  to  extreme  and  sudden 
changes  of  temperature,  as  the  unequal  expansion 
and  contraction  of  glass  and  metal  may  cause  the 
cement  between  the  lenses  to  crack.  Also  keep  them 
from  direct  sun-light. 

Screw  them  into  the  nose-piece  and  unscrew,  by 
grasping   the   milled  edges. 

Avoid  any  violent  contact  of  the  front  lens  with 
the  cover-glass.  Usually  the  latter  suffers,  but  it  is 
as   liable    to   occur  to  the  former. 

Above  all,  it  should  be  made  a  rule  that  no  one 
but  the  owner  handle  the  microscope  and  aeces 
sories.  One  person  may  be  expert  in  the  manipu- 
lation of  one  instrument  and  still  find  it  difficult  to 
work  with  another.  The  fine  adjustment  particularly 
causes  the  greatest  difficulty  as  in  some  instruments 
it  corresponds  with  the  movement  of  the  microme- 
ter screw,  while  in  others  it  is  contrary  and  thus 
the    objective    as    well    as    object    are   endangered. 


APPENDIX. 


(Re-printed  from  "The  Microscope,"  Jan.,  1885.) 


CONSIDERATIONS     IN     TESTING     OBJECTIVES. 


EDW.     BAUSCH. 


There  is  a  laudable  desire  in  almost  all  persons 
possessing  a  microscope  to  become  intimately  ac- 
quainted with  it,  and  for  this  pur])ose  it  is  not 
only  necessary  to  learn  the  use  of  its  mechanical 
parts,  but  to  understand  its  optical  capacity, 
which  is  considerably  more  difficult,  and  which  in- 
volves more  considerations  than  would  appear  on 
first   thought. 

With  all  the  care  which  may  be  bestowed  upon 
objectives,  they  are  to  a  certain  extent,  works  of 
chance,  and  depend  upon  the  optician's  judgment, 
industry  and  skill,  and  upon  the  variations  in  glass, 
for  their  excellence  and  uniformity.  These  condi- 
tioas  are    often    so   varying   that   in    the   case  of  sey- 


90 

eral  objectives  of  the  same  formula,  made  at  the 
same  time  there  will  be  such  great  differences  that 
it  can  hardly  be  conceived  on  the  first  examin- 
ation, that  they  were  to  be  similar.  It  is  at 
this  point  especially  necessary  to  detect  the  errors, 
to  determine  their  cause  and  apply  the  remedy, 
and  do  this  properly  often  involves  an  inconceiva- 
ble amount  of  work,  and  in  many  cases  the  final 
results  are  reached  at  a  pecuniary  loss.  There  are 
certain  fixed  tests  for  each  kind  of  objective,  and 
to  the  best  of  my  knowledge  all  reputable  opticians 
bring  each  objective  up  to  its  standard  before  allow- 
ing it  to  pass  their  hands,  irrespective  of  the  cost 
of  doing  so.  This  must  of  necessity  be  so,  if  only 
out  of  business  consideration,  and  not  for  a  love 
of  each  production,  for  it  is  evident  that  a  well- 
earned  reputation  would  soon  lose  its  pre-emmence, 
and  would  acquire  one  for  unreliable  or  poor  work, 
if  on  comparison,  objectives  of  the  same  kind  would 
show  a  marked  difference.  There  is  sometimes  •  a 
fortunate  combination  of  circumstances  which  makes 
a  certain  objective  better  than  its  fellows,  but  this 
is  a  rare  exception,  and  is  positive  evidence  that 
the  exact  requirements  of  the  formula  have  been 
complied  with.  As  a  rule,  therefore,  I  believe  that 
the  opticians'  claim  may  be  relied  upon,  and  where 
the  results  in  the  hands  of  the  microscopist  do  not 
correspond  with  them,  the  cause  may  usually  be 
looked   for   in  the    lack    of    experience    in    manipula- 


dl 


tion  or  in  conditions,  which  differ  from  those  under 
which  the  objective  was  completed.  The  beUef, 
which  I  am  aware  is  extant,  that  there  are  ^reat 
differences  in  objectives  purporting  to  be  similar,  is. 
in  my  opinion,  not  justified,  at  any  rate  in  the 
productions  of  those  men  who,  by  general  acknowl- 
edgement, are  at  the  head  of  their  profession.  I 
admit  that,  as  in  everything  which  depends  upon 
human  skill,  there  is  strictly  speaking,  no  absolute 
uniformity,  but  also  claim,  that  with  few  exceptions, 
the  differences  are  so  shght,  that  anything  but  the 
most   expert    manipulation    cannot    detect   them. 

It  therefore  appears  to  the  writer  that  any  in- 
forrhatioH  which  will  tend  to  improve  the  knowledge 
of  testing  objectives  will  not  only  prove  beneficial 
to  the  microscopist,  but  will  prove  advantageous  to 
the  optician,  in  that  his  work  will  receive  a  fair 
trial,  based  upon  a  knowledge  of  the  principles  in- 
volved, and  that  he  may  be  convinced  that  all  his 
work  which  deserves  commendation  will  be  the  bet- 
ter appreciated.  The  following  points  are  by  no 
means  new,  but  are  often  lost  sight  of  in  making 
tests.  The  writer  will  speak  of  medium  and  high 
power  objectives  only,  as  the  deleterious  influences 
are  most  noticeable  in  these,  but  they  apply  as 
well    to    the    lower    powers    though    in    a    less    degree. 

The  part  of  the  instrument  which  has  a  strong 
bearing  in  the  performance  of  the  objective  is  the 
mirror.     It  should    be    adjustable    on   the    mirror  •  bar, 


•       92 

so  that  it  can  be  accommodated  to  the  variations 
in  distance  of  the  source  of  light  from  the  instru- 
ment. When  parallel  rays  are  used,  as  with  light 
from  the  sun  or  clouds,  its  distance  from  the  ob- 
ject should  be  decreased  and  increased  when  lamp- 
light is  used.  It  should  be  exacted  that  the  focus 
of  the  concave  mirror  be  within  the  Hmits  of  its 
adjusti*nent.  The  serious  disadvantage  of  its  incorrect- 
ness in  this  respect  can  easily  be  seen  by  taking,  for 
instance,  a  1-5  objective  which  will  resolve  P.  aiigu- 
latiim  nicely  with  central  light,  when  the  mirror  is 
exactly  focused.  By  moving  the  latter  out  of  focus 
it  will  be  seen  that  the  objective  loses  in  per- 
formance, and  if  this  is  carried  sufficiently  far  it  will 
arrive  at  a  point  where  the  objective  will  cease  to 
show  any  lines.  The  effect  will  be  the  same  on 
any  other  object,  and  is  caused  by  the  lack  of 
proper  concentration  of  light  on  the  slide.  When 
oblique  light  is  used,  unless  the  diaphragm  moves 
with  the  mirror,  it  should  be  removed,  as  the  ad- 
vantage of  obliquity  is  diminished  or  destroyed  by 
the    loss    of  light. 

The  cover  glass  exerts  probably  the  greatest  in- 
fluence in  testing  as  well  as  in  general  work  This 
should  be  used  of  a  thickness  which  corresponds  to 
that  to  which  the  objective  ( if  non  adjustable )  was 
originally  corrected.  If  thicker  or  thinner  covers  be 
used,  the  objective  will  be  spherically  over  or  under 
corrected,  and  will  have  to  be  moved  correspond- 
ing* above    or    below   the  plane   ( outline )    of  the  ob- 


93 

ject  to  distinguish  its  structure,  if  the  variation  is 
considerable  the  difference  between  the  two  planes 
will  be  so  great  that  it  will  cease  to  show  any 
structure,  and  it  may  then  be  said  to  be  lacking 
in  defining  power  although  in  reaHty  it  possesses  it, 
but  is  not  properly  used.  Generally  speaking  the 
objective  may  be  said  to  be  spherically  corrected 
when  it  gives  the  best  defined  image  ;  that  is,  when 
the  outline  and  internal  structure  of  an  object  of 
extreme  thinness  appear  in  one  plane.  When, 
after  the  objective  has  been  focused  on  the  out- 
line of  the  object,  it  is  necessary  to  increase  the 
distance  to  focus  on  the  structure,  it  is  evidence 
that  the  objective  is  sperically  over-corrected  and 
that  the  cover  is  too  thick;  in  adjustable  objectives 
the  correction  collar  must  be  brought  to  its  closing 
point,  which  means  that  the  lenses  are  brought  in 
closer  contact.  When  the  objective  must  be  focused 
to  a  point  beyond  the  outHne  of  the  object  to  see 
its  structure;  that  is,  brought  closer  to  the  cover- 
glass,  it  proves  that  this  is  too  thin,  and  is  then  said 
to  be  spherically  under-corrected  ;  to  give  the  proper 
adjustment  in  an  adjustable  objective  in  this  case 
the  adjustment  is  opened — the  lenses  are  separated. 
It  requires  a  certain  amount  of  study  to  distinguish 
these  phenomena,  and  although  it  can  be  done  in 
well  prepared  specimens,  I  know  of  none  better  than 
coarsely  marked   diatoms,    such    as    P.    angidatiuii. 

Although    I    am    aware    that    many    eminent    micro- 
scopists    do  not    favor    adjustable    objectives  for  every 


94 

day  work,  I  must  confess  that  I  fail  to  see  the 
force  of  their  arguments.  From  the  foregoing  it 
will  be  seen  that  unless  the  cover-glasses  are  of  a 
thickness  corresponding  to  that  which  was  originally 
used,  the  objective  may  be  made  to  do  imperfectly 
what  is  in  its  power  to  do  well,  and  when  pressed 
to  its  full  capacity  may  and  is  likely  to  fail.  It 
must  be  remembered  that  cover-glasses  of  the  same 
number  are  not  of  the  same  thickness.  The  selec- 
tion of  those  of  proper  thickness  is  expensive  and 
tedious,  whereas  the  knowledge  of  correcting  the 
objective  is  easily  acquired,  and  in  the  latter  case 
it  is  in  the  manipulator's  power  to  command  the 
highest  performance  of  which  the  objective  is  capa- 
ble; further  than  this,  it  has  the  advantage  that  it 
may  be  used  as  a  non-adjustable  objective  if  de- 
sired. When  homogeneous  nnmersion  objectives  were 
first  introduced  they  were  mounted  in  fixed  settings, 
as  it  was  expected  that  the  thickness  of  the  cover- 
glass  would  not  affect  the  correction ;  although  this 
assumption  was  correct,  it  was  found  that  even  in 
these  it  was  necessary.  How  much  more  then,  is  it 
required  in  dry  or  water  immersion  objectives  ? 

Another  factor  in  the  disturbing  influences  is  the 
variation  in  length  of  tube  ;  the  deleterious  results 
are  similar  to  those  with  varying  cover-glasses.  Ob- 
jectives are  usually  adjusted  to  8^  or  9  inches 
length  of  tube,  and  although  this  in  itself  is  a  fixed 
standard,  it  usually  becomes  variable  by  changing 
objectives   and   eye-pieces.     That  this   is   so  in  objec- 


95 


tives  is  patent,  and  that  it  is  so  in  eye-pieces  can 
easily  be  determined  by  making  a  change  in  pow- 
ers, when  it  will  be  found  that  a  change  in  focus 
is  required.  By  decreasing  the  length  of  tube  the 
objective  will  appear  to  be  spherically  under- cor- 
rected and  vice  versa  when  it  is  increased,  so  that"  it 
is  apparent  that  by  the  use  of  the  draw-tube  the 
effect  of  the  cover-glass  may  be  partially  neutralized  ; 
for  instance,  when  by  the  use  of  a  thin  cover  the 
objective  is  spherically  under-corrected,  it  may,  to 
a  certain  extent,  be  corrected  by  causing  a  corre- 
sponding over-correction  in  the  tube  by  increasing 
its  length.  The  use  of  the  draw-tube  for  the  pur- 
pose of  changing  the  amplification  or  for  the  matter 
of  convenience  can  hardly  be  commended,  except  in 
cases  where  adjustable  objectives  are  used. 

Considerable  also  depends  upon  the  perfection  of 
the  eye-piece.  I  believe  that,  as  a  rule,  too  little 
care  is  devoted  to  it;  at  any  rate,  it  is  certain  that 
while  any  Huyghenian  eye-piece  for  a  telescope  can 
be  used  on  a  microscope,  very  few  which  have  been 
made  for  this  can  be  used  on  a  telescope ;  and 
while  it  is  true  that  no  such  perfection  may  be 
required  in  the  former,  it  leaves  such  an  indefinite 
range  that  it  may  become  difficult  to  place  a  limit 
for  the  perfect  and  imperfect.  In  all  work,  and  espe- 
cially in  testing,  it  should  be  seen  that  the  eye-lens, 
as   well   as    the    field-lens,    are   perfectly   clean, 


96 


Among  the  absolutely  necessary  conditions  in  judg- 
ing of  the  quality  of  an  objective  are  perfect  speci- 
mens, especially  if  they  are  sections.  A  thick  ob- 
ject obstructs  the  light  and  generally  makes  it 
necessary  to  go  through  so  many  layers  or  planes 
that  it  is  difficult  to  get  any  one  distinct  ;  the  im- 
pression may  thus  easily  be  given  that  the  objective 
is  at  fault.  The  difference  between  two  objects  of 
the  same  nature  may  be  so  great  that,  while  with  one 
the  objective  may  be  condemned  as  imperfect,  it  may 
with  the  other  appear  to  be  of  extraordinary  excel- 
lence. 

In  conclusion,  I  will  say  that  there  may  be  other 
conditions  which  may  influence  the  performance  of  a 
lens,  and  to  acquire  the  power  of  eliminating  them 
requires  considerable  experience.  When  an  objective 
does  not  correspond  with  the  claims  of  the  optician, 
judgment  should  not  be  passed  upon  it  until  after 
repeated  trials  have  been  made,  in  all  of  which  the 
above  points  should    not   be   lost   sight  of. 


THE 


BADSCH  Si  LOMB  OPTICAL  CO. 


HAVE   THE   MOST 


IMPROVED  MECHANICAL  FACILITIES. 


AND   ARE   THEREFORE    ABLE   TO    MAKE 


ANY    KIND    OF 


Special    Apparatus 


AND 


Mechanical  Deirices 


IN   ANY    DESIRED   QUANTITY. 


They  will    furnish    ESTIMATES  on   receipt  of 
Drawings  and    Descriptions. 


On  the  following  pages  will  he  found 
Illustrations  of  a  iiumhei'  of  Instru- 
ments, manufactured  hij  the  BAUSCH 
S>    lOMB  OPTICAL  CO. 

For  complete  descriptions  of  Micr\j- 
scopes,  as  well  as  OBJECTIVES  and 
ACCESSORIES,  send  for  complete  cata- 
logue. 

Address  : 

P.  0.  Box  85 Jf,  Piochester,  ^'.  T. 
P.  0.  Box  JfS2,  New  York. 


^^^ 


EXCELSIOR   DISSECTING   MICROSCOPE. 


COMPACT   DISSECTING   MICROSCOPE. 


FAMILY    MICROSCOPE. 


HARVARD    MICROSCOPE. 


MODEL   MICROSCOPE. 


niiiis 


INVESTIGATOR    MICROSCOPE. 


IMPROVED    MICROTOMES. 


THE  MICROSCOPE 

AN  ILLUSTRATED  MONTHLY  JOURNAL. 


EDITORS   AND   PUBLISHERS: 

C.  H.  STOWELL,  M.  D.,  F.  R.  M.  S., 

Prof,  of  Histology  and  Microscopy  in  the  TJnlver&tty  of  Michigan. 

LOUISA  REED  STOWELL,  M.  S.,  F.  R.  M.  S., 

Assistant  in  Microscopical  Botany,  Univ'ers'ty  of  Michigan. 


Price  $1.00  a  Year  for  the  United  States:    5  shillings  for 

Great  Britain. 

PUBLISHED  AT  ANN  ARBOR,  MICH. 


'•That  Dr.  and  Mrs.  Stowell  have  in  them  the  elements  necessary  for 
successful  journalism  is  apparent."— Mich.  Med.  News. 

"  The  names  of  the  editors  alone  is  ample  guarantee  to  insure  continued 
success  to  this  journal."— Good  Health. 

•'  It  fills  a  sphere  of  its  own  and  should  be  in  the  hands  of  every  physi- 
cian and  druggist  ifi  the  country."— John  Phin,  in  Am.  Jouk.  of  Mic. 

"  It  alwavs  has  a  neat  appearance  and  is  constantly  filled  with  good  mat- 
ter."—Sanitary  News. 

"It  Improves  steadily  and  decidedly  with  every  issue."— Bistottrt. 

"It  Is  producea  in  an  excellent  form,  is  well  printed,  and  illustrated 
with  good  illustrations.  '—Science. 


Twenty-two  of  the  twenty-four  pages  of  tlie  January  number 
for  1885,  consisted  of  ORIGINAL  COMMUNICATIONS; 
while  the  February  number  consisted  ENTIRELY  OF  ORIG- 
INAL COMMUNICATIONS,  with  the  exception  of  two 
small  items.  In  these  two  numbers  alone  there  were  OVER 
TWENTY  original  communications.  The  other  numbers  of 
the  year  have  not  appeared  at  this  writing,  but  the  peculiar  fea- 
ture of  having  the  columns  filled  with  the  latest  and  best  original 
matter  will  be  continued  through  the  year. 

Address, 

C-    131.    STO"\A7^ELL3 

Ann  Arbor,  Mich. 


AMERICAN  MONTHLY 


ICROS 


J 


n 


P 


AL  JOU[[ML, 


EST-A-BLISHEID     1880. 


AN    ILLUSTRATED    PERIODICAL 


FOR   ALL   WHO    USE 


THE     MICROSCOPE 

EITHER  PROfESSIOKALLY  OR   A3   AMATEURS. 


TESTIMONIALS. 

Extracts  from  letters  of  well-known  workers  with  tiie  micro- 
scope. These  being  unsolicited  and  not  intended  for  publication, 
the  signatures  are  withheld. 

•*I  hope  the  Journal  will  continue  to  prosper,  and  that  its  columns  in 
the  future,  as  in  the  past,  will  be  filled  v^ith  hints  and  suggestions  to  working 
microscopists,  both  amateur  and  professional." 

"I  enjoy  your  Journal  very  much.  I  learn  many  good  things  from  it, 
and  I  am  sure  that  is  the  case  with  all  who  read  it." 

"  I  want  to  thank  you  for  your  excellent  Journal.  I  am  pleased  and  sat- 
isfied and  I  want  you  to  know  it." 

"  I  send  you  ,$1.00  enclosed,  and  wish  you  much  success  for  the  new  year 
and  you  richly  deserve  it. 

"I  am  very  well  pleased  with  th^  Journal  and  its  standing,  considering 
the  wide  range  of  subjects  it  has  to  cover.  But  there  is  one  feature  about 
your  Journal  that  cauuot  be  praised  too  high.y.  ♦  «  ♦  Although  tbey 
might  yield  a  nice  income  to  the  editor,  I  tniuk  this  disinterestedness  should 
be  generally  appreciated." 

"1  am  glad  to  see  by  the  last  number  that  its  present  condition  and  fu- 
ture prospects  are  satisfactory;  and  for  myself  can  say  that  It  Is  aiway.s  re- 
ceived and  perused  with  pleasure." 


SUBSCRIPTION  PRICE,  $1  PER  YEAR, 

STRICTLY   IN   ADVANCE. 
Editor  and  Publisher, 

R.  HITCHCOCK, 

P.  O.  Box,  630.  Wasliington,  D.  C. 


THE  STUDENT'S 


MANUAL  OF  HIS 


m 


1  \J 


J 


Y 


FOR   THE   USE   OF 


STUDENTS,    PEACTITIOXEES    AND    MICROSCOPISTS. 


BY 


CHAS.    H.    STOWELL-   M.  D., 


ANN   ARBOR,    MICH. 


Price    $3.50. 


iPEi^sonsrs  ^^x&x'j?xi<r<3r 


TM^'  'W®ffiS 


^ 


iv-w 


(fi^ 


ARE  CORDIALLY  INVITED  TO  CALL  AT  OUR 


BRANCH   OFFICE, 

37  Maiden  Lane, 


WHERE  WE  KEEP   ON   HAND 


OF  ALL  ARTICLES  MANUFACTURED  BY  US. 


BAUSCH  &  LOMB  OPTICAL  CO. 


^i^ 


mmmm. 


mmm 


